1) You could try pressing "ctrl + alt + backspace" to kill the X server. If that does not work, try pressing "ctrl + alt + F[1-6]", that will bring you up to the command line where you can manually kill the Xserver.
2) You could start checking your logs. /var/log/xfree (or something similar to that) could give you some ideas of what have happened. Most important logs are located at /var/log, so take a look