Were you running GNOME as your desktop? What is the full error message? If your at the command, you can type df, and see if your disk is full, that sometimes can clear up this problem by clearing some disk space.
Or try starting the system in single-user mode (or running the command 'telinit 3' to switch to the "normal" multi-user mode without any display manager (graphical login) running. Then remove the PID file and any stray core files in the root, /root and similar directories.
Or you can try to reconfigure X by using Xconfigurator to see if that gets you back into X windows.