I had this problem when i enabled 3d effects on my server. The screen just goes white and you cannot see anything. Even after a reboot as soon as you logon to gnome the screen goes white.
To fix you’ll need to logon using a “failsafe terminal” session, then run gnome-cleanup from the command line. This will blow away any of your gnome settings (although they are actually backed up to a file), but it’ll mean you’ll be able to logon again without issue.