I feel your pain!
I am having a lot of problems with a multi-screen setup myself
Check if there is a backup of your xorg.conf file in /etc/X11/
Replace the current xorg.conf with the backup
sudo cp /etc/X11/name.of.backup /etc/X11/xorg.conf
If not, try
If you make any changes to xorg.conf you have to restart your display manager.
You can stop and start the display manager (for testing) by doing the following:
alt+F1 for terminal
alt+F8 should bring you back to graphical display
sudo /etc/init.d/gdm stop
sudo /etc/init.d/gdm start