Try the command /unstick - it should move you back to a more valid position. If that doesn't work, try /who game and see if any GMs are online. Send him/her a tell if you see one and ask that you be moved back just a bit. Unfortunately if no GMs are on and /unstick won't do anything, you'll have to go with /die. That command will send your character to the Death Realm, and you'll have to find the exit - usually following other people does the trick.
