The very first step is to hold down the key and using canned air, blow a few short blasts below and above the key. This would clear any unseen debris that may hamper the key movement.

I strongly suspect my first assessment is correct, though. Just too many hard hits on that particular key, which resulted in pad failure. It happens. Here are a couple links that will make the job a lot easier:

http://psrtutorial.com/music/projects/contacts.html

http://psrtutorial.com/music/projects/ContactStrips.html

Good luck,

Gary