Sometimes the GRUB bootloader gets messed up, and you get dumped into the EFI Shell. A few key commands to get things working again…
map gets a list of all the discovered partitions
FS0: gets you to the file structure of the FS0 partition
bcfg boot dump -b lists all the EFI boot entries
bcfg boot mv 6 0 changes boot order
bcfg boot add 0 FS0:\EFI\debian\grubx64.efi “GRUB” adds an entry