Restoring Grub2 from the booting cd-rom

In case of Debian Squeeze,
after going into the install steps, proceed until loading the device drivers.
Then, if pushed Ctrl+Alt+F2, you can get the command prompt.

# fdisk -l
# mount -t ext4 /dev/XXX /chroot
# mount --bind /dev /chroot/dev
# mount -t sysfs none /chroot/sys
# mount -t proc none /chroot/proc
# chroot /chroot/
# grub-install /dev/XXX