try this:
sudo dpkg-reconfigure xserver-xorg
You may get an error asking you about tour video bus. This is a good thing. It means it did rewrite the file. If this happens, then attempt to reload gdm:
/etc/init.d/gdm
then restart and see what happens.
Here, it can get a bit complicated. The next bits are only if you have a problem. IF you end up with a blank screen that you can't do anything on, then ctrl+alt+F1. This will take you to a command line. This time, reconfigure the graphics card at a lower resolution...sstart off maybe 800X600.
Once you have reconfigured, then restart X (you don't have to reboot, just type:
sudo /etc/init.d/gdm restart
or alternately, you could choose to:
sudo /etc/init.d/gdm stop
sudo /etc/init.d/gdm start
Should work then. If not, you'll have to check your /etc/X11/xorg.conf file and look for the horizontal and vertical refresh of your monitor. Got to Monitor section and the part your looking for will be labled as:
HorizSync 30-107
VertRefresh 48-120
If this isn't there, or the values afterwards are blank, then you will have to enter those entries and/or values for your specific monitor by hand. This information is generally found in your owners manual or the manufacturers web site.
ALTERNATIVELY
You could bring up synaptic and reinstall x.