Which key?
How is the key "not working"? Sticking/ repeating/not sending the key, etc..
Spread out a towel.
Unplug the keyboard and turn it upside down over the towel.
Gently but firmly tap the ends and sides with the palm of your hand. Likely that lots of crud will fall out. Especially if you eat while working or playing on the computer.
With any luck you may dislodge some piece of debris from around the key and all will be well.
Failing that: pry off the key cap and clean. That is more risky but could be worth the try.
Nothing to lose with the keyboard not usable anyway.....