A good screw extractor set should be able to get it out if you can get to the bolt reasonably. To use a screw extractor, you would drill a hole as concentricly as possible in the shaft of the sheared off bolt and insert the extractor. The extractor consists of a tool steel shaft with high angle reversed threads on it. As you rotate (lefty loosy) the tool the threads dig into the bolt and should grip it enough that it will begin to turn and back out.