Cable may be stripped, or a gear, or you may have played the cable out. Probably something stripped. If you need to get your roof down you will want to get into the gears (in the headliner section behind the sunroof) that drive the lifting arms and remove them. Hovever, then your sunroof won't lock and that is a hazard in itself. Check out
http://www.pelicanparts.com/techarticles/944_Sunroof/944_sunroof.htm for a good overview.