may be a problem if you cant get it out. is there enough left sticking out where you can clamp onto it with a pair of vice grips. make sure you heat it up throughly with some mapp gas or equivalent before you try and twist it.
edit: if you cant grab it with vice grips you can cut a slit in the head of the stud with a cut-off wheel and try and turn it out with a screw driver. once again heat it up before you try to twist it out.