I've got a similar problem - installed Zorin Core alongside Windows 8.1. I got the option to boot into Zorin or Windows on the first re-boot and never since - only Windows boots now. Disabled Fast Boot and Secure Boot. Also booted from USB and ran the Boot Repair w/o luck. It generated an error file but it's all greek to me 
In BIOS, no Zorin options are shown in boot priority and the F12 boot manager only shows the Windows option.
Error file contents:
boot-repair-4ppa200-zorin1 [20230331_0523]
============================= Boot Repair Summary ==============================
ping: google.com: Temporary failure in name resolution
Recommended repair: ____________________________________________________________
The default repair of the Boot-Repair utility will reinstall the grub-efi of
sda6,
using the following options: sda2/boot/efi
Additional repair will be performed: unhide-bootmenu-10s use-standard-efi-file
Mount sda2 on /mnt/boot-sav/sda6/boot/efi
Unhide GRUB boot menu in sda6/etc/default/grub
======================== Reinstall the grub-efi of sda6 ========================
chroot /mnt/boot-sav/sda6 grub-install --version
grub-install (GRUB) 2.04-1ubuntu26.16+zorin1
chroot /mnt/boot-sav/sda6 modprobe efivars
chroot /mnt/boot-sav/sda6 efibootmgr -v before grub install
BootCurrent: 0002
Timeout: 0 seconds
BootOrder: 2001,0001,2002,2003
Boot0000* ubuntu HD(2,GPT,0514028c-0381-4f6a-a8f9-0fa04e309d5a,0x12c800,0x96000)/File(EFIubuntushimx64.efi)
Boot0001* Windows Boot Manager HD(2,GPT,0514028c-0381-4f6a-a8f9-0fa04e309d5a,0x12c800,0x96000)/File(EFIMicrosoftBootbootmgfw.efi)WINDOWS.........x...B.C.D.O.B.J.E.C.T.=.{.9.d.e.a.8.6.2.c.-.5.c.d.d.-.4.e.7.0.-.a.c.c.1.-.f.3.2.b.3.4.4.d.4.7.9.5.}....................
Boot0002* USB HDD: SanDisk PciRoot(0x0)/Pci(0x1d,0x0)/USB(0,0)/USB(0,0)/HD(1,MBR,0x56d356f9,0x24c,0x2100)RC
Boot0003* Unknown Device: HD(2,GPT,0514028c-0381-4f6a-a8f9-0fa04e309d5a,0x12c800,0x96000)/File(EFIubuntushimx64.efi)RC
Boot2001* EFI USB Device RC
Boot2002* EFI DVD/CDROM RC
Boot2003* EFI Network RC
chroot /mnt/boot-sav/sda6 uname -r
5.15.0-56-generic
chroot /mnt/boot-sav/sda6 grub-install --efi-directory=/boot/efi --target=x86_64-efi
Installing for x86_64-efi platform.
Installation finished. No error reported.
Error: no grub*.efi generated for Zorin OS 16.2 (16). Please report this message to boot.repair@gmail.com
chroot /mnt/boot-sav/sda6 efibootmgr -v after grub install
BootCurrent: 0002
Timeout: 0 seconds
BootOrder: 0000,2001,0001,2002,2003
Boot0000* ubuntu HD(2,GPT,0514028c-0381-4f6a-a8f9-0fa04e309d5a,0x12c800,0x96000)/File(EFIubuntushimx64.efi)
Boot0001* Windows Boot Manager HD(2,GPT,0514028c-0381-4f6a-a8f9-0fa04e309d5a,0x12c800,0x96000)/File(EFIMicrosoftBootbootmgfw.efi)WINDOWS.........x...B.C.D.O.B.J.E.C.T.=.{.9.d.e.a.8.6.2.c.-.5.c.d.d.-.4.e.7.0.-.a.c.c.1.-.f.3.2.b.3.4.4.d.4.7.9.5.}....................
Boot0002* USB HDD: SanDisk PciRoot(0x0)/Pci(0x1d,0x0)/USB(0,0)/USB(0,0)/HD(1,MBR,0x56d356f9,0x24c,0x2100)RC
Boot0003* Unknown Device: HD(2,GPT,0514028c-0381-4f6a-a8f9-0fa04e309d5a,0x12c800,0x96000)/File(EFIubuntushimx64.efi)RC
Boot2001* EFI USB Device RC
Boot2002* EFI DVD/CDROM RC
Boot2003* EFI Network RC
chroot /mnt/boot-sav/sda6 update-grub
Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/init-select.cfg'
Generating grub configuration file ...
Found theme: /usr/share/grub/themes/zorin/theme.txt
Found linux image: /boot/vmlinuz-5.15.0-69-generic
Found initrd image: /boot/initrd.img-5.15.0-69-generic
Found linux image: /boot/vmlinuz-5.15.0-56-generic
Found initrd image: /boot/initrd.img-5.15.0-56-generic
grub-probe: error: cannot find a GRUB drive for /dev/sdb1. Check your device.map.
Found Windows Boot Manager on /dev/sda2@/EFI/Microsoft/Boot/bootmgfw.efi
Adding boot menu entry for UEFI Firmware Settings
Unhide GRUB boot menu in sda6/boot/grub/grub.cfg
An error occurred during the repair.
Error: no grub*.efi generated for Zorin OS 16.2 (16). Please report this message to boot.repair@gmail.com
You can now reboot your computer.
Please do not forget to make your UEFI firmware boot on the Zorin OS 16.2 (16) entry (sda2 file) !
If your computer reboots directly into Windows, try to change the boot order in your UEFI firmware.
If your UEFI firmware does not allow to change the boot order, change the default boot entry of the Windows bootloader.
============================ Boot Info After Repair ============================
=> No boot loader is installed in the MBR of /dev/sda.
sda1: __________________________________________________________________________
File system: ntfs
Boot sector type: Windows 8/10/11/2012: NTFS
Boot sector info: No errors found in the Boot Parameter Block.
Operating System:
Boot files:
sda2: __________________________________________________________________________
File system: vfat
Boot sector type: Windows 8/10/11/2012: FAT32
Boot sector info: No errors found in the Boot Parameter Block.
Operating System:
Boot files: /efi/Boot/bootx64.efi /efi/Boot/fbx64.efi
/efi/Boot/mmx64.efi /efi/ubuntu/grubx64.efi
/efi/ubuntu/mmx64.efi /efi/ubuntu/shimx64.efi
/efi/ubuntu/grub.cfg /efi/Microsoft/Boot/bootmgfw.efi
/efi/Microsoft/Boot/bootmgr.efi
/efi/OEM/Boot/bootmgfw.efi /efi/OEM/Boot/bootmgr.efi
sda3: __________________________________________________________________________
File system:
Boot sector type: -
Boot sector info:
sda4: __________________________________________________________________________
File system: ntfs
Boot sector type: Windows 8/10/11/2012: NTFS
Boot sector info: No errors found in the Boot Parameter Block.
Operating System: Windows 7
Boot files: /Windows/System32/winload.exe
sda5: __________________________________________________________________________
File system: ntfs
Boot sector type: Windows 8/10/11/2012: NTFS
Boot sector info: No errors found in the Boot Parameter Block.
Operating System:
Boot files:
sda6: __________________________________________________________________________
File system: ext4
Boot sector type: -
Boot sector info:
Operating System: Zorin OS 16.2
Boot files: /boot/grub/grub.cfg /etc/fstab /etc/default/grub
sdb: ___________________________________________________________________________
File system: iso9660
Boot sector type: Unknown
Boot sector info:
Mounting failed: mount: /mnt/BootInfo/FD/sdb: /dev/sdb already mounted or mount point busy.
================================ 2 OS detected =================================
OS#1: Zorin OS 16.2 (16) on sda6
OS#2: Windows 7 on sda4
================================ Host/Hardware =================================
CPU architecture: 64-bit
Video: HD Graphics 5500 GM108M [GeForce 840M] from Intel Corporation NVIDIA Corporation
Live-session OS is Zorin 64-bit (Zorin OS 16.2, focal, x86_64)
===================================== UEFI =====================================
BIOS/UEFI firmware: V1.14(1.4) from Insyde Corp.
The firmware is EFI-compatible, and is set in EFI-mode for this live-session.
SecureBoot disabled - SecureBoot disabled - Please report this message to boot.repair@gmail.com.
BootCurrent: 0002
Timeout: 0 seconds
BootOrder: 0000,2001,0001,2002,2003
Boot0000* ubuntu HD(2,GPT,0514028c-0381-4f6a-a8f9-0fa04e309d5a,0x12c800,0x96000)/File(\EFI\ubuntu\shimx64.efi)
Boot0001* Windows Boot Manager HD(2,GPT,0514028c-0381-4f6a-a8f9-0fa04e309d5a,0x12c800,0x96000)/File(\EFI\Microsoft\Boot\bootmgfw.efi)WINDOWS.........x...B.C.D.O.B.J.E.C.T.=.{.9.d.e.a.8.6.2.c.-.5.c.d.d.-.4.e.7.0.-.a.c.c.1.-.f.3.2.b.3.4.4.d.4.7.9.5.}....................
Boot0002* USB HDD: SanDisk PciRoot(0x0)/Pci(0x1d,0x0)/USB(0,0)/USB(0,0)/HD(1,MBR,0x56d356f9,0x24c,0x2100)RC
Boot0003* Unknown Device: HD(2,GPT,0514028c-0381-4f6a-a8f9-0fa04e309d5a,0x12c800,0x96000)/File(\EFI\ubuntu\shimx64.efi)RC
Boot2001* EFI USB Device RC
Boot2002* EFI DVD/CDROM RC
Boot2003* EFI Network RC
64349b3622c65f495a99dbf6102496e3 sda2/Boot/bootx64.efi
a5e9b71b5ba86166bee504e1b254b7cf sda2/Boot/fbx64.efi
f75c300397e73f3fc7bfe46d49819bb2 sda2/Boot/mmx64.efi
78dce18613524e5ea5d6059f51b7178f sda2/ubuntu/grubx64.efi
f75c300397e73f3fc7bfe46d49819bb2 sda2/ubuntu/mmx64.efi
64349b3622c65f495a99dbf6102496e3 sda2/ubuntu/shimx64.efi
3827b6fa1f4022001328be9d79e33b18 sda2/Microsoft/Boot/bootmgfw.efi
baefbe270eb996b3b093e4866fbd6333 sda2/Microsoft/Boot/bootmgr.efi
6c1910730f135cbd5a78e3a48520e647 sda2/OEM/Boot/bootmgfw.efi
154eb317efa335209708260b0b946eb7 sda2/OEM/Boot/bootmgr.efi
============================= Drive/Partition Info =============================
Disks info: ____________________________________________________________________
sda : is-GPT, no-BIOSboot, has---ESP, not-usb, not-mmc, has-os, has-win, 2048 sectors * 512 bytes
Partitions info (1/3): _________________________________________________________
sda1 : no-os, 32, nopakmgr, no-docgrub, nogrub, nogrubinstall, no-grubenv, noupdategrub, not-far
sda2 : no-os, 32, nopakmgr, no-docgrub, nogrub, nogrubinstall, no-grubenv, noupdategrub, not-far
sda4 : is-os, 32, nopakmgr, no-docgrub, nogrub, nogrubinstall, no-grubenv, noupdategrub, farbios
sda5 : no-os, 32, nopakmgr, no-docgrub, nogrub, nogrubinstall, no-grubenv, noupdategrub, farbios
sda6 : is-os, 64, apt-get, signed grub-pc grub-efi , grub2, grub-install, grubenv-ok, update-grub, farbios
Partitions info (2/3): _________________________________________________________
sda1 : isnotESP, part-has-no-fstab, no-nt, no-winload, recovery-or-hidden, no-bmgr, notwinboot
sda2 : is---ESP, part-has-no-fstab, no-nt, no-winload, no-recov-nor-hid, no-bmgr, notwinboot
sda4 : isnotESP, part-has-no-fstab, no-nt, haswinload, no-recov-nor-hid, no-bmgr, notwinboot
sda5 : isnotESP, part-has-no-fstab, no-nt, no-winload, recovery-or-hidden, no-bmgr, notwinboot
sda6 : isnotESP, fstab-has-goodEFI, no-nt, no-winload, no-recov-nor-hid, no-bmgr, notwinboot
Partitions info (3/3): _________________________________________________________
sda1 : not--sepboot, no---boot, part-has-no-fstab, not-sep-usr, no---usr, part-has-no-fstab, no--grub.d, sda
sda2 : not--sepboot, no---boot, part-has-no-fstab, not-sep-usr, no---usr, part-has-no-fstab, no--grub.d, sda
sda4 : not--sepboot, no---boot, part-has-no-fstab, not-sep-usr, no---usr, part-has-no-fstab, no--grub.d, sda
sda5 : not--sepboot, no---boot, part-has-no-fstab, not-sep-usr, no---usr, part-has-no-fstab, no--grub.d, sda
sda6 : not--sepboot, with-boot, fstab-without-boot, not-sep-usr, with--usr, fstab-without-usr, std-grub.d, sda
fdisk -l (filtered): ___________________________________________________________
Disk sda: 931.53 GiB, 1000204886016 bytes, 1953525168 sectors
Disk identifier: 92DDFE1C-3AC4-4F3B-96C7-8029D8D23DE3
Start End Sectors Size Type
sda1 2048 1230847 1228800 600M Windows recovery environment
sda2 1230848 1845247 614400 300M EFI System
sda3 1845248 2107391 262144 128M Microsoft reserved
sda4 2107392 1338943347 1336835956 637.5G Microsoft basic data
sda5 1920405504 1953523711 33118208 15.8G Windows recovery environment
sda6 1338943488 1920405503 581462016 277.3G Linux filesystem
Partition table entries are not in disk order.
Disk sdb: 231.38 GiB, 248437014528 bytes, 485228544 sectors
Disk identifier: 0x56d356f9
Boot Start End Sectors Size Id Type
sdb1 * 0 6050239 6050240 2.9G 0 Empty
sdb2 588 9035 8448 4.1M ef EFI (FAT-12/16/32)
sdb3 6053888 485228543 479174656 228.5G 83 Linux
parted -lm (filtered): _________________________________________________________
sda:1000GB:scsi:512:4096:gpt:ATA WDC WD10JPVX-22J:;
1:1049kB:630MB:629MB:ntfs::hidden, diag;
2:630MB:945MB:315MB:fat32::boot, esp;
3:945MB:1079MB:134MB::Microsoft reserved partition:msftres;
4:1079MB:686GB:684GB:ntfs:Basic data partition:msftdata;
6:686GB:983GB:298GB:ext4::;
5:983GB:1000GB:17.0GB:ntfs::hidden, diag;
sdb:248GB:scsi:512:512:msdos:SanDisk Cruzer Glide 3.0:;
2:301kB:4626kB:4325kB:::esp;
3:3100MB:248GB:245GB:ext4::;
Free space >10MiB: ______________________________________________________________
sdb: 4.41MiB:2956MiB:2952MiB
blkid (filtered): ______________________________________________________________
NAME FSTYPE UUID PARTUUID LABEL PARTLABEL
sda
├─sda1 ntfs ACC6B7E1C6B7A9CA fc8867fb-1aad-4bb1-a799-609ff80c42ac Recovery Basic data partition
├─sda2 vfat AABA-0C9A 0514028c-0381-4f6a-a8f9-0fa04e309d5a ESP EFI system partition
├─sda3 5994ed39-b029-46ae-a3fd-0314233fbb0c Microsoft reserved partition
├─sda4 ntfs 7C92BB9A92BB56FC 31c27a18-af92-4366-9f9c-1db7b13dc925 Acer Basic data partition
├─sda5 ntfs EE56BCD356BC9E31 2df6c35e-5498-4dd7-9870-0338283ab2dc Push Button Reset Basic data partition
└─sda6 ext4 ad3dabe7-1266-4e30-908c-c5bd8624a399 927c7e34-981d-4f2c-b555-0da9dbaafbf5
sdb iso9660 2022-12-08-19-53-13-00 Zorin OS 16.2 Core 64bit
├─sdb1 iso9660 2022-12-08-19-53-13-00 56d356f9-01 Zorin OS 16.2 Core 64bit
├─sdb2 vfat 0A35-FF80 56d356f9-02
└─sdb3 ext4 cd251068-cdb6-473c-8fd4-c3ba0b3cf394 56d356f9-03 writable
Mount points (filtered): _______________________________________________________
Avail Use% Mounted on
/dev/disk/by-label/writable[/install-logs-2023-03-31.0/crash] 212.4G 0% /var/crash
/dev/disk/by-label/writable[/install-logs-2023-03-31.0/log] 212.4G 0% /var/log
/dev/sda1 328.4M 45% /mnt/boot-sav/sda1
/dev/sda2 239.8M 19% /mnt/boot-sav/sda2
/dev/sda4 269.8G 58% /mnt/boot-sav/sda4
/dev/sda5 1.9G 88% /mnt/boot-sav/sda5
/dev/sda6 246.4G 4% /mnt/boot-sav/sda6
/dev/sdb1 0 100% /cdrom
Mount options (filtered): ______________________________________________________
/dev/disk/by-label/writable[/install-logs-2023-03-31.0/crash] ext4 rw,relatime
/dev/disk/by-label/writable[/install-logs-2023-03-31.0/log] ext4 rw,relatime
/dev/sda1 fuseblk rw,relatime,user_id=0,group_id=0,allow_other,blksize=4096
/dev/sda2 vfat rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro
/dev/sda4 fuseblk rw,relatime,user_id=0,group_id=0,allow_other,blksize=4096
/dev/sda5 fuseblk rw,relatime,user_id=0,group_id=0,allow_other,blksize=4096
/dev/sda6 ext4 rw,relatime
/dev/sdb1 iso9660 ro,noatime,nojoliet,check=s,map=n,blocksize=2048,iocharset=utf8
===================== sda2/efi/ubuntu/grub.cfg (filtered) ======================
search.fs_uuid ad3dabe7-1266-4e30-908c-c5bd8624a399 root hd0,gpt6
set prefix=($root)'/boot/grub'
configfile $prefix/grub.cfg
====================== sda6/boot/grub/grub.cfg (filtered) ======================
Zorin ad3dabe7-1266-4e30-908c-c5bd8624a399
Zorin, with Linux 5.15.0-69-generic ad3dabe7-1266-4e30-908c-c5bd8624a399
Zorin, with Linux 5.15.0-56-generic ad3dabe7-1266-4e30-908c-c5bd8624a399
Windows Boot Manager (on sda2) osprober-efi-AABA-0C9A
### END /etc/grub.d/30_os-prober ###
UEFI Firmware Settings uefi-firmware
### END /etc/grub.d/30_uefi-firmware ###
========================== sda6/etc/fstab (filtered) ===========================
# <file system> <mount point> <type> <options> <dump> <pass>
# / was on /dev/sda6 during installation
UUID=ad3dabe7-1266-4e30-908c-c5bd8624a399 / ext4 errors=remount-ro 0 1
# /boot/efi was on /dev/sda2 during installation
UUID=AABA-0C9A /boot/efi vfat umask=0077 0 1
/swapfile none swap sw 0 0
======================= sda6/etc/default/grub (filtered) =======================
GRUB_DEFAULT=0
GRUB_TIMEOUT_STYLE=menu
GRUB_TIMEOUT=10
GRUB_DISTRIBUTOR=lsb_release -i -s 2> /dev/null || echo Debian
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""
GRUB_THEME=/usr/share/grub/themes/zorin/theme.txt
GRUB_DISABLE_OS_PROBER=false
==================== sda6: Location of files loaded by Grub ====================
GiB - GB File Fragment(s)
638.458015442 = 685.539074048 boot/grub/grub.cfg 1
643.216430664 = 690.648383488 boot/vmlinuz 2
643.216430664 = 690.648383488 boot/vmlinuz-5.15.0-56-generic 2
645.624938965 = 693.234499584 boot/vmlinuz-5.15.0-69-generic 1
647.172733307 = 694.896431104 boot/initrd.img 3
647.172733307 = 694.896431104 boot/initrd.img-5.15.0-56-generic 3
649.060493469 = 696.923398144 boot/initrd.img-5.15.0-69-generic 1
647.172733307 = 694.896431104 boot/initrd.img.old 3
===================== sda6: ls -l /etc/grub.d/ (filtered) ======================
-rwxr-xr-x 1 root root 18214 Aug 13 2022 10_linux
-rwxr-xr-x 1 root root 42349 Aug 13 2022 10_linux_zfs
-rwxr-xr-x 1 root root 12894 Aug 13 2022 20_linux_xen
-rwxr-xr-x 1 root root 12059 Aug 13 2022 30_os-prober
-rwxr-xr-x 1 root root 1424 Aug 13 2022 30_uefi-firmware
-rwxr-xr-x 1 root root 700 Jul 3 2022 35_fwupd
-rwxr-xr-x 1 root root 214 Aug 13 2022 40_custom
-rwxr-xr-x 1 root root 216 Aug 13 2022 41_custom
=========================== sda6/etc/grub.d/35_fwupd ===========================
#! /bin/sh
SPDX-License-Identifier: LGPL-2.1+
set -e
[ -d ${pkgdatadir:?} ]
shellcheck source=/dev/null
. "$pkgdatadir/grub-mkconfig_lib"
if [ -f /var/lib/fwupd/uefi_capsule.conf ] &&
ls /sys/firmware/efi/efivars/fwupd-*-0abba7dc-e516-4167-bbf5-4d9d1c739416 1>/dev/null 2>&1; then
. /var/lib/fwupd/uefi_capsule.conf
if [ "${EFI_PATH}" != "" ] && [ "${ESP}" != "" ]; then
echo "Adding Linux Firmware Updater entry" >&2
cat << EOF
menuentry 'Linux Firmware Updater' $menuentry_id_option 'fwupd' {
EOF
${grub_probe:?}
prepare_grub_to_access_device '${grub_probe} --target=device \${ESP} | sed -e "s/^/\t/"'
cat << EOF
chainloader ${EFI_PATH}
}
EOF
fi
fi
