I had a similar problem (although I have a laptop) and a friend of mine (whose job is repairing computers) helped me fix it. Here's what you can try:
Unplug the keyboard and push down every key. Not one after the other, but apply pressure to all keys, many times, and then let it rest for a few minutes. Plugging it back afterwards, it may or may not be fixed (that's a suggestion like any other, that may not be your issue). If that's the issue, it will discharge the capacitors and the key will be "un-stuck".