You try pressing ctrl+alt+F1 to see if it gives you a command prompt. If that doesn't work, try booting with a Knoppix CD. You can mount your root partition and chroot over to it:
mount /dev/hda1 /mnt/hda1
where "hda1" is your root partition, whatever that might be. After that you will be running as root on your Debian machine and you can enter whatever commands you like to reconfigure X windows.