If the stud is broken close to the face of the head you may be able to use a cutoff wheel and make a grove in the stud. This will allow you to use a straight screw driver and grab what is left of the stud minus what you have already drilled from the center. This solution works best before you do any drilling but you may get lucky depending on how much of the stud you drilled. I have been there before. Whatever you do don't breakoff the easy out or tap then you will be screwed. Good luck!