Use needle-nose pliers to grip any exposed end of the broken key and pull it straight out
Apply a small amount of lubricant to the keyway
Use a broken key extractor tool to hook and slide the key fragment out
Insert a thin jigsaw blade or hacksaw blade with the teeth facing outward to catch the key fragment
Gently wiggle the tool while pulling the fragment toward the opening
If the fragment is flush or deep inside, tap the lock lightly while trying to extract it
If the key will not move, remove the lock cylinder if possible and work from the back
If extraction fails, call a locksmith