Things You Will Need
WD-40 or a spray lock lubricant, needle nose pliers, coping saw or scroll saw blade or a very thin jigsaw blade
Spray the lock with the WD-40 or lock lubricant so the key will move easier.
The vast majority of keys are thinner on one side. Insert the saw blade into the lock next to the key on the thin side. Make sure the teeth on the blade are facing out (towards you).
Turn the teeth of the saw blade slightly towards the key until it grabs the teeth of the key.
Pull the blade and key gently out a little until there is enough showing to grab with the needle nose pliers. Note: if the key won't move you may have to turn the key in the lock slightly using the saw blade to get it to a neutral position before you can pull on it.
When the key is out enough, grab it with the needle nose pliers and pull it the rest of the way out.
Calling a locksmith can prove to be expensive if you have broken a key in the lock cylinder.
Tips & Warnings
If needle nose pliers are not available where you are nail clippers or tweezers can work almost as well.