Booting from grub shell
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
I had the following config
/dev/hda2 - / /dev/hda3 - /boot
To boot this from the grub shell
root=(hda0,2)
This points to /boot (hd0 is /dev/hda - the number after the comma is zero indexed partition number - so ,2 is /dev/hda3)
kernel=/vmlinuz-... root=/dev/hda2 ro
Grub shell has tab completion to help you find the correct name for the kernel file
initrd=/initrd...
Again - tab complete to get the correct name for the initrd image
boot
Once you're in - then the first thing to run as root is
update-grub