You mean youhave access to the GRUB console, or the GRUB directory?
What you need to do is quite simple; just run :
$sudo fdisk /devhda
maybe nano will not work, in that case ,use vi. then modify your menu.lst accordingly to what is your root partition. If you don't know how to do that, just post the output of both commands here, and I'll try to talk you through it.
P.S. : The Debian update may have left the old kernel, try booting that.