Grub is the bootloader used on Ubuntu.
grub_file_filters not found
After upgrading Ubuntu from Bionic to Focal (20.04), I received the error
symbol 'grub_file_filters' not found.
To solve this, I adjusted the boot drive order in the BIOS settings to boot from
/boot partition, as per this StackOverflow
solution. Others have suggested running
update-grub using a live CD, but the boot drive order seems to be an easier
thing to try first.