Comment 37 for bug 1848797

I faced this issue while upgrading a cloud-based system from 18.04 to 20.04.

Naturally I had created a snapshot of the system before attempting the upgrade it, so when the upgraded system failed to boot, and after having read much of what may have caused it here and elsewhere, I restored the snapshot, re-performed the upgrade, and before rebooting, I re-installed grub and issued an update-grub command.

I'm blind guessing that in my case it had to do with the mapping of the virtual system volume between the guest system and its host/hypervisor. Somewhere in the upgrade grub misses where it should install/update itself.