Several things can be wrong. The drive belt should not come off on its own. There is a key on the crankshaft that makes the pulley on the crankshaft turn. It could be sheared. The belt could be too worn to work anymore. They do wear and the wedge shaped profile gets too thin. There is a spring from the mower deck to the transmission that can break. The cable from the handle to the transmission has a dogleg on the end that engages the clutch in the trans. That too can break.

Just found the site and can see how helpful it can be. I have a 215 and just discovered the belt was off the drive pulley. It does not seem obvious on how to get it back on without taking the thing apart. What did you wind up doing after your last post.