The mechanism is seized, probably due to a leaking washer pipe at some point.
You need to get the rear panel off through the boot; not an easy job but doable. One screw in the top of each of the handles then pull like mad. You'll then be able to pop the boot manually, get to all of the mechanism and free it up.
I've just had to do mine but luckily the key barrel didn't seize up so I could get in there