Remove a screw with a broken head
Knock out the drywall around it until you can grab it with locking pliers. Either unscrew it or break it off (if it broke going in it may not want to come out any more cooperatively.) Spackle the hole and move whatever it is that caused you to drill it up or down a little bit so the new hole does not line up with the old one if the screw is broken off in place.