When you install grub to debian and forget to run update-grub the first time before you reboot then there is no menu.lst file in /boot/grub so you boot to the grub shell
Full details on
http://myrddin.org/howto/debian-grub.php
If you have LILO on the MBR then the grub-install device is the disk e.g. /dev/hda - not the root partition.