You'll need to mount the root partition (usually under /mnt/) and then you'll need to make sure the right system partitions are bind mounted to your newly mounted root partition:
Code:
mount -o bind /dev /mnt/dev
mount -o bind /proc /mnt/proc
mount -o bind /sys /mnt/sys
After this, you should be able to run the needed commands to reinstall grub properly (not sure what they are since I use lilo myself).
