Well, assuming the piece of bolt is still in the captured nut, you can take an easy-out to it and extract it, then thread a new bolt into the nut. You can drill out the bolt (assuming it won't come free in a less-destructive manner) and heli-coil the hole back to its original size. Assuming there is enough meat in the captured nut, you could drill out the bolt and tap to a larger size, too.