Pressione ctrl
+ alt
+ F1
para entrar no console tty1 e faça o login.
Execute os seguintes comandos
sudo apt-get remove --purge fglrx fglrx-amdcccle fglrx-dev
sudo apt-get remove --purge xserver-xorg-video-ati xserver-xorg-video-radeon
sudo apt-get install xserver-xorg-video-ati
sudo apt-get install --reinstall libgl1-mesa-glx libgl1-mesa-dri xserver-xorg-core
sudo mv /etc/X11/xorg.conf /etc/X11/xorg.conf.backup
sudo rm -rf /etc/ati