I was trying to reinstall GRUB to use the new kernel image I compiled, but it was failing:
# grub-install /dev/sda
Fortunately using the “grub” terminal solved it:
# grub grub> root (hd0,0) root (hd0,0) Filesystem type is ext2fs, partition type 0x83 grub> setup (hd0) setup (hd0) ... Done.