Remote locking transmits signal in 2 ways: Radio signal and light signal.
In your case, the radio signal has stopped and you are using the light signal. I believe the light signal only works on the driver's side. If you find that you can not lock/unlock the door, just clean the glass cover of the sensor on the driver's door.
Several things can cause the fault. I believe the fuse controlling it is located in teh fuse bank under the back seat.
