Find a USB keyboard, just to test it. If everything's working well with external (it should) then it's best to open up the laptop and re-seat the keyboard cable. If this doesnt help you may need a new keyboard. Also, sense the movement/resistance of the C-key. Is it going all the way down, does it make the same click as all other working keys? It may just be a case of dirt being under the key, not allowing it to go all the way down, or the rubber pad to be misaligned.
Hope this helps.