Broken Key
If you can see a piece of the key sticking out of the lock, start by lubricating it with a bit of oil or WD-40. WD-40 is ideal because its long, thin nozzle allows you to spray directly near the key. Carefully spray the oil into the lock, between the key and the cylinder. Don't hesitate to use plenty of oil. Then, use a pair of pliers to pull out the broken key.