There has been a production issue on the keys whereby the connections on the battery have failed. This will kill the key.
Another issue affects the buttons on the fob, 1 or all 3 may become inoperative.
Both failures require the fob to be replaced and a fob programmed, as Mat said, take your spare key with you as current software doesn't allow the programing of a single key. All keys need to be erased and reprogrammed.
A less common concern could also be due to a poor connection at the remote receiver, This little box is fitted behind the roof lining.
remove the front interior light and look though. you'll see a small black box stuck to the metal work (velcro
) pull it off and through the interior light hole in the headliner.
with it dangling off the wiring, open the box up but don't touch the PCB
press a button on the fob and you should see a small red led come on with each press.
repeat the button press for each of the buttons on the fob.
if the operation of the led is a bit flaky, disconnect the wiring and replug it, try the buttons again....