Possibly a bad central locking pneumatic pump, it's under the rear seat.
Open the right rear window and stand over near the open window and try locking/unlocking the doors and listen for the pump to run. If it's working, possibly the pneumatic hose connector has come off, either that or the pump/motor is junk.
If it doesn't run, then it could be anything, bad fuse, a switch someplace, who knows? Or the pump is REALLY junk and won't even try running.

Gilly