Just had a big problem with my at-work PC. I upgraded from Intrepid to Jaunty, and X stopped working. Not just a little - hard lock on start. UG! Tried previous kernels, no go, same symptom, so not that.

Tried recovery mode netroot, root with networking. didn't know my graphics card. Looked at xorg.conf for complexities. But, xorg.conf yielded NOTHING, it was very generic, it can't be failing. Running lspci shows I'm running ATI RV515 GL (that's a FireGL V3350 card). Jaunty upgrade failed with this card.

Link Here to read great explanation of fixing it

Summary of commands from this other page:

$ sudo apt-get remove –purge xorg-driver-fglrx 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
$ dpkg-reconfigure -phigh xserver-xorg

Presto, reboot and glorious!