Well, not good - - 1. You might try using a "heat "gun" and heat the area around the stud.  (The Aluminum head will expand)
2. Get some good penetrant and apply, hoping it will soak in aroung the threads.
3. Find some quality Vise-Grips and use what shank you have left and try to turn out the stud.
4. If this fails - you may have to carefully drill the center of the stud and use a good easy-out.
5. If it still refuses to come out, you will have to drill out the stud and rethread to proper size..
Not much magic here, just patience and luck..