If you can access the other end of the sheared off bolt you might consider cutting a slot in it and screwdrivering it out.
Another method is to drill the broken end and insert a special little tool called an easy-out which you screw into the drilled hole anticlockwise and continue turning when it bites into the sheared bolt.
http://static.zoovy.com/img/usfreight/W250-H250-Bffffff/easyout5.jpg
Good luck...
Peccavi
June 2009