1. Press Ctl-Alt-F1. This should take you to the screen you launched X from. Do you have a command prompt? You might get it if you hit Enter a couple of times.
2. If this fails press Ctlr-Alt-F2. This will take you to another terminal. Log in.
You can then shut down from there. More importantly try
That will show you what is running and possibly give you a clue about what is wrong.