Oh boy that might be messed up.... try this:
Start it up until you get to GDM
This should take you to tty4
Log in with the same administrative user name you normally use.
Then, one at a time, waiting for it to finish:
Then CTRL+ALT+F7 to return to GDM. Try logging in and see what happens. If it doesn't work, return to tty4 and to reboot it to see if it works now.
sudo aptitude update
sudo aptitude upgrade
sudo aptitude dist-upgrade
Beyond that, I can't help you much, Ubuntu runs like Ubuntu and has forked a bit from Debian.
You can check your sources.list from tty4 by:
CTRL+X to get out of it after viewing.
sudo nano /etc/apt/sources.list