You need to centre punch a mark on sheared stud, then drill a pilot hole in centre and use a stud extractor on it, such as these
http://www.ebay.co.uk/itm/DRAPER-EXPERT-SCHRODER-5-PIECE-SCREW-EXTRACTOR-SET-45856-BOLT-STUD-REMOVER-/150898560701?pt=UK_Hand_Tools_Equipment&hash=item2322414ebd
make sure you give it a good soaking with wd40 or equivalent, Although heat from drilling might loosen it a bit.