It could be a missing EFI Boot entry. Can you boot from an USB drive check the output from
fdisk -l /dev/sda
fdisk -l /dev/sdb
fdisk -l /dev/nvme0n1
?
– one of those is probably the USB stick … but the other could be your Qubes OS drive. If one of your drives looks like:
Device Start End Sectors Size Type
/dev/nvme0n1p1 2048 1230847 1228800 600M EFI System
/dev/nvme0n1p2 1230848 3327999 2097152 1G Linux filesystem
/dev/nvme0n1p3 3328000 976773119 973445120 464.2G Linux filesystem
and you and do:
mount /dev/nvme0n1p1 /mnt/
ls /mnt/EFI/qubes/grubx64.efi
then you should be able to add an EFI boot entry with:
efibootmgr -c -d /dev/nvme0n1 -p 1 -L "QUBES" -l '\EFI\qubes\grubx64.efi'