Can't you get a terminal with Ctrl+Alt+f1?
If you can

Code:
sudo apt-get install --reinstall libgl1-mesa-glx libgl1-mesa-dri xserver-xorg-core
Code:
sudo apt-get install --reinstall libgl1-mesa-glx libgl1-mesa-dri xserver-xorg-core
and finally:

Code:
sudo apt-get install xserver-xorg-video-ati
should get you up and running. Provided ofcourse that libglapi did install correct since that was the problem.