Your belt is probably broken. First UNPLUG YOUR DRYER! Lift the front edge of the top. There are two clips holding it down in front. You will be able to see the top of the drum, and the belt around it, if it isn't broken. Then remove the front by removing the two sheet metal screws (one on each side) and lift the front off. Pull the drum out, and vacuum all lint and dust out. If the belt is broken, replace it. If not, it's probably the plastic idler pulley. Replace your parts and reassemble.
Paul
January 2009