GDM refuses to start after updates or accidental driver removal

GDM will fail to start if there is no corresponding device driver.

The actual reason for the failure of GDM to start can be figured out by checking /var/log/Xorg.0.log. Try to start GDM manually and verify the logs to get the reason. If the reason is that unable to load driver library, open /etc/X11/xorg.conf and comment the line with "Driver" keyword in it(Make sure that you are commenting out in the appropriate section). Now start gdm and it should start.

The concept here is that when there is no device driver we should make linux kernel to take one from its defaults.

After gdm has started, you can install the required drivers from hardware managers.

This worked for Ubuntu 10.04.

More details here