Sounds like the key barrel is seized. Open your boot and remove the trim from inside the tail gate (below the rear window). Locate the key barrel and try pushing and pulling on the rod that connects to it. If there is no movement give it a good soaking with WD40. If this fails to sort it remove the key barrel and try working it loose. If it still wont work replce it with a new one or try a breaker to get one cheaper.
Simon Mayes, October 2005