Before you do anything, try opening the bottom case (very very easy and undetectable later) and checking the screws that attach the trackpad button - there are three screws on each side, one on the metal spring that attaches to the actual button on top and the other securing that metal 'spring' to the base. It is possible that the single screw has fallen out and, if not, you'll probably see what the problem is just by looking around there.