Chances are if you were using a faceplate and the screw broke, you are in the process of removing the faceplate to hollow the inside...could be wrong on assumption, this is usually just where mine break. Then hopefully you shaped the outside AND have turned a foot or a tenon to grab the bowl with the chuck. If this is the case, then mount the bowl on the chuck, hollow out as close as you can to the broken screw on each side, and grab the screw with plier such as vise grips.