It is part of the central locking mechanism. Try unlocking your door, and see if the filler door then opens. If not, and your door locks work (if not, the door should be in the open default position anyway), try taking the six or so screws securing the panel cover over the locking actuator, and observe wether the actuator has failed by locking and unlocking the doors. If not, look for a new actuator, as I've not been able to find anyone who makes a key lock gas cap for our model. Good luck!