did u spill anything on it? if so you should be able to drip some 90% alcohol in there and cycle the button a bunch to try and free it up. be sure if you do this to unplug the batter and let it sit for a minute then do the alcohol. and then take some canned air to blow it out and let it sit for a while to let the alcohol evaporate out to keep it from shorting anything out.
if nothing was spilled in there it might have just given out and will need to be replaced.
if its warrantied still take it in. if not then youll have to take the hit to the wallet.