If you mean the physical "click" sensation has disappeared, it might be your battery reaching the end of its life. What can sometimes happen is that the battery, which is below the trackpad, begins to swell up as it wears out. This can put pressure on the trackpad from behind, the typical symptom of which is the "click" gets shallower and shallower and then stops working. (I believe the trackpad "click" can be adjusted, too, so it could simply be the case that it's out of adjustment.)
I've personally seen this on two machines, a polycarbonate Macbook and a pre-unibody aluminium Macbook Pro, both with (fortunately) removable batteries. It also happens to batteries from other manufacturers - it's not an Apple-specific problem, nor especially unexpected, just a characteristic of lithium polymer batteries that they swell up when they begin to fail.
I've not personally seen this on a MacBook Air or any unibody Macbook Pro, and there might be something special the newer machines' batteries which prevents this happening, given that the batteries are designed not to be removed or replaced by the user. I wouldn't bet on it, though. You might want to do some web searches to see if anyone else appears to be having problems with expanding batteries in the same model.
If you suspect that this is the issue (and I would certainly suspect it if the physical "click" has gone from the trackpad), then I cannot in conscience recommend that you try to replace the battery yourself - Apple don't consider the battery to be user-replaceable part, and lithium polymer batteries can be (very) dangerous if mishandled or incorrectly installed. However, if it is expanding, you need to have it removed ASAP or it will distort and damage the machine, possibly irreparably.
If in doubt, have an apple centre check over your machine at the earliest opportunity. They'll be able to tell you if the trackpad needs adjusting or the battery needs replacing. Battery replacement is likely to be fairly expensive unless it's an unexpected early failure replaceable under warranty. (I've found Apple to be good about replacing parts which have failed unexpectedly.)
Most manufacturers warranties explicitly exclude batteries, considering them essentially a consumable. I'm not sure what Apple's position is for the ones which are designed not to be removed, though, they can hardly tell you simply to buy a new battery. I would personally expect them to replace it within the warranty period, and for a reasonable fee outside of the warranty period, but I don't know what their policy is.
A third party repairer should be able to replace a battery if no other damage has been done.
