I'm back at my Linux computer now. I was on a public XPee machine before. With X running I have, in /tmp, a file called .X0-lock with the PID number and a directory .X11-unix with =X0 in it.
Also, this happened occasionally with KDE, I don't know if it affects GNOME, but I would get the "unable to open display" error if my ~/.ICEauthority was corrupt. Delete it so it can be recreated.
Looking back at the errors, did you check the ownership and permission for /dev/agpgart?
i was downloading some files from net...and the screen got locked...i got stuck
Another possibility, maybe you didn't lock the screen yourself. Maybe your disk got full so X can't write the temporary files and logs that it needs to. Check that with df -h / and df -h /tmp
and the screen got locked.when i tried 2 press enter r space bar it didn't ask for password.instead it didn't show anything in the screen
Next time try Ctrl+Alt+BkSpc to kill X without shutting down.
Hope this helps.