The usual suspects for this crime is something was spilled into the keyboard: Liquid of some kind or food crumbs. I've seen staples as well as paper mess-up a key or two. The last possibility is the battery as sometimes they swell up pressing from the bottom.
For crumbs: Use a can of can'ed air you could try blowing in small bursts along the keys (don't let the liquid spray out!).
For a spill: You could pop the keycaps off and carefully clean underneath them hopefully saving the expense of replacing the full keyboard.
For a swelled battery: Its time to replace! To see if thats it you'll need to pop off the bottom and loosen it. Does the keyboard work then?