Is any of it sticking out past the block, or is it broke off flush or slightly inside the block?
If there is still some sticking out, you might try welding a nut on the end of it, waiting for it to cool and then try to turn it out with a wrench.
If it's too close for welding, grind or file it down flush, center puch it DEAD CENTER of the stud and STRAIGHT into it with a small drill bit. Use a slightly bigger bit and enlarge the hole. Keep using bigger bits until you to get to a little over half of the stud size, then try an easy-out on it. Be careful though . . . a broken off easy-out is a REAL problem.