If you can access another terminal (press ctrl+alt+f1 keys) type sudo sh /usr/share/ati/fglrx-uninstall.sh
.
That is the official driver remover from ATI, after that use the "Additional Drivers" application to install the ubuntu packages.
After you reboot to a clean desktop make sure all the old fglrx packages are gone:
sudo apt-get purge fglrx*
Remove your xorg.conf
sudo rm /etc/X11/xorg.conf
Reinstall xorg
sudo apt-get install --reinstall xserver-xorg-core libgl1-mesa-glx:i386 libgl1-mesa-dri:i386 libgl1-mesa-glx:amd64 libgl1-mesa-dri:amd64
Configure Xorg
sudo dpkg-reconfigure xserver-xorg
Reboot:
sudo reboot
After you get to your desktop again use the "Additional Drivers" program but don't enable the post update ones, use the previous version.