Go back into your bios & make sure that nvme0n1 (if that is where grub is installed) is first in the bootup order.
If it is & you still have issues have a look here > GRUB Disappears as soon as Boot USB is removed
Go back into your bios & make sure that nvme0n1 (if that is where grub is installed) is first in the bootup order.
If it is & you still have issues have a look here > GRUB Disappears as soon as Boot USB is removed