I have a triple-boot machine (Lenovo T430, Windows/Zorin/Ubuntu Mate. The Ubuntu Mate installation is encrypted.
Yesterday I booted into Zorin OS and ran its updates. Unfortunately, after a reboot the Ubuntu Mate option was missing from the menu. I have attempted to use Boot Repair, but it refuses on the basis that it can't read the encrypted partition, even after unlocking per their instructions .
Can anyone point me in the right direction to restore this option in my GRUB menu?
Does the upgrade process take any sort of backup/snapshot during the update process?
 sudo cryptsetup luksOpen /dev/sdb2 myvolume
I am wondering if the metadata header is damaged?
"cryptsetup is used to conveniently setup dm-crypt managed device-mapper mappings. These include plain dm-crypt volumes and LUKS volumes. The difference is that LUKS uses a metadata header and can hence offer more features than plain dm-crypt. On the other hand, the header is visible and vulnerable to damage."
(Source: cryptsetup - manage plain dm-crypt and LUKS encrypted volumes | linux commands examples)
I always followed the advice given by Open SuSE 9.3 Professional, "warning, encrypting partitions can lead to data loss". The same applies to Windows. My manager wanted her Business Officer to have data made secure. (This was in the time of Windows XP).
The very next day the computer crashed and Windows had to be reinstalled meaning the encrypted partition inaccessible because fresh installation means new hash keys are created. Fortunately the Business Officer had made a backup of all critical data so not all was lost, just one days worth of work. All I can suggest for now is backup any data you can access and may have to look at installing Timeshift after a fresh install. No guarantees, but you could try and install EasyBCD from Neosmart and see if you can create bootloader in Windows to access your MATE partition.