Exact same thing happened to me at the weekend, I sent my key to Chris on here for repair ( he has ebay listings for key repair). Key should hopefully be back tomorrow, he has refurbed the key as he couldn't find any fault with it.
I tried my spare to make sure the car was picking the signal up from the key, to prove its a key problem.
This is the advice Chris kindly gave me to try before I sent my key off to him, maybe its worth a try for you, hopefully he doesn't mind me sharing:-
If the key suddenly stopped try resynchronising first as this may be the problem.
*Open the car manually (the alarm may go off). Turn on the ignition without starting the car (position MAR) and wait for the code light on the instrument panel to go out, which takes a few seconds.
* Press the unlock/disarm button for at least 10 seconds.
If this does not work:
*Stand near the car to make sure the car can receive the signal properly. If you can sit in the car without the alarm sounding then do this.
* Remove the battery from the key to interrupt the power (i.e. slip out the battery holder part-way) and then reinsert and try the key button again, twice. Take your time and perform this procedure slowly.
You may have to do this battery removal and re-insertion quite a few times to reset the key enough to have an effect - 35 is the max I have ever heard from a customer but this was exceptional, less than 10 times is much more common.
If your remote locking system is generally working but misbehaving and acting oddly:
* Disconnect the car battery for 20 minutes. Ensure the battery is fully charged and in good health. This is the most common cause of erratic remote alarm/locking behaviour.
If it still doesn't work then you either have a fault on the key which I can help you with, or you may need a professional recoding doing. As I am a lot cheaper than a recode and will refund if their is nothing to repair then I suggest you send the key to me first.