sounds like the 'castle' inside the lock has bust, you can get door lock repair kits on ebay, just make sure you get the right type for your key, its eiither a laser cut type or the normal tooth key type.
or just take the handle off and take a look at it, one torq bolt on the inside of the door shut, and the handle will come out complete with lock.
and if your not that bothered- you can actually flip driver and passanger handle over!