1 Answer

Because keys can stick for many different reasons, you need to do two things:
1. Diagnose the problem by taking the fallboard (key cover) out of your piano and inspecting the key and how it connects to the jack and hammer.* Compare the how the mechanism behaves to other surrounding keys. Isolate and test the ease of motion of all of the moving parts in the key mechanism one by one until you find the culprit. It could be something as simple as a piece of paper or a guitar pick stuck between the keys.
2. If you can't figure out what's wrong, then call a certified piano technician. If you're lucky, and the fix is an easy one, the technician may not even charge you! (This is so that you'll think of him/her when you need a piano tuning or some more major repair.)
3. (I know that I told you to do *two* things.) Though I don't recommend it, you *could* just do nothing. Especially if you don't run your A/C all the time, the humidity in the summer air could have made the wooden parts swell up. There's a good chance that the problem will just go away in a few months. I have a squeaky soft pedal on my piano. It only squeaks from July through October, so I just ignore it.

*If you can't figure out how to take the key cover out, you need to look it up on the internet. Every brand of piano has a different mechanism for removing the fallboard. Just google the words: fallboard, remove, and the name of your piano manufacturer