Comment 2 for bug 360565

I was wrong about the computer being totally locked up. Only the console is locked up. I was able to ssh in from another box. Running X under gdb gives:
/usr/bin/X: symbol lookup error: /usr/lib/xorg/modules/extensions//libdri.so: undefined symbol: atiddxAbiDixLookupPrivate

This is a duplicate of bug #345669 or bug #285603

Following the instructions in bug #285603 fixed the problem:
sudo apt-get remove --purge fglrx* xserver-xorg-video-ati xserver-xorg-video-radeon
sudo apt-get install xserver-xorg-video-ati
sudo service gdm restart