[matey]

Usually you can pres control alt and F1 thru F6 and get into the terminal screen.
then you can log in and do a ps ax to see the processes, kill the proc. number (very first numbers on the left).
you may have to do
ps ax |more
to see all processes.

btw kill the proc related to your GUI.
Also once you get into the terminal screen you can re config with this command;

sudo dpkg-reconfigure xserver-xorg
or

sudo dpkg-reconfigure -phigh xserver-xorg
(or man xorg.conf ) to read the manual

To take the GUI from starting permanently do;

update-rc.d -f gdm remove (or kdm or xdm whichever youre using)

to put it back into startup;

update-rc.d -f gdm default (or kdm or xdm instead of gdm)

good luck!

BTW2 hopefully someone with Linux experience will solve this problem. I am not an expert.