1994 Ford Probe Ignition switch

If a bit of the broken key is sticking out of the lock, grasp it firmly with needle-nose pliers and pull it out.

If the broken end of the key is visible but you can't grasp it with pliers, try the following superglue method.
Use a toothpick to apply a tiny amount of superglue to the broken edge of the key's head.
Insert the head into the lock so the broken edges of the key come together. Be extremely careful not to let the glue come into contact with any part of the lock.
Hold the key in place firmly for at least 3 minutes to allow the glue to dry and bond the broken pieces.
Carefully extract the key from the lock. Avoid unnecessary twisting or turning because this will strain the superglue bond.

Call a locksmith if you can't extract the key with either method. Locksmiths have a key-extractor tool designed specifically for this predicament.