You could try using Super Grub2 Disk on a USB stick (I recommend using Ventoy so you can have multiple ISOs on one USB without having to re-flash to try something else) to see if that can find your Zorin install and boot you into it. If you can, then run efibootmgr to see what (U)EFI operating systems are registered on your machine, and set the correct one (Zorin?) as default. Old and mis-configured OSes can hang around in the (U)EFI register and mess things up if you're not careful.
Example output from my current PC:
efibootmgr
BootCurrent: 0002
Timeout: 1 seconds
BootOrder: 0002,0003,0000,000B,000C,000F,0001,0004,0005
Boot0000* Windows Boot Manager HD(1,GPT,99f821a7-628b-4c84-bdd6-8c7407152b42,0x800,0x82000)
Boot0001* UEFI:CD/DVD Drive BBS(129,,0x0)
Boot0002* Manjaro HD(1,GPT,99f821a7-628b-4c84-bdd6-8c7407152b42,0x800,0x82000)
Boot0003* Fedora HD(1,GPT,99f821a7-628b-4c84-bdd6-8c7407152b42,0x800,0x82000)
Chances are you installed Zorin either after another 32bit grub was on your system, or Zorin 15 still supported 32bit systems and your subsequent Zorin updates luckily didn't break grub by failing to install a 64bit version 