A couple of possibilities here. The first is that your car may have lost the programming. This would be unusual, but a possibility none the less. Do you have any remotes that work?
I'm assuming none of the buttons work? These are virtually identical in construction to television remote controls. The inside can become dirty and prevent contact when the buttons are pressed. What I've done in the past (with TV remotes) is taken them apart, cleaned the rubber half with a very mild soap, dabbed it dry then allowed it to thoroughly air dry. Then, using a clean pencil eraser (preferably a large one, not on the end of a pencil), gently clean the corresponding contacts on the printed circuit board. These are coated differently that computer PCB'* (this process also works on gold plated edge connectors), but will do the trick. GENTLY rub the contacts with the eraser. Brush away the eraser remnants with something like a Q-Tip.
It'* also possible that your remote has died altogether.
StingRay Bike w/cards in the spokes, Weeeeeee!