Several things to look for. First, take off the top belt cover and inspect the belt and the pulley on the top of the trans. Often times that belt, if allowed to get out of adjustment, (via the adjustment knob on the traction cable) will jump off the pulley that's on the engine and get wedged between the pulley and the engine. To check "that" or replace the belt, it is a pain because you need to remove the blade, blade hub (ugh), and crank protector plate. Those belts are special belts. They are smaller than 1/2" and I think they are close to 3/8"
If when the belt cover is off, rotate the trans pulley by hand (CW) and make sure you see the output shaft of the trans going "backwards" AND both wheels have traction from the pinion gears. (you might have to slightly lift the machine to make sure both wheels are rotating. If they are not, remove the wheels and check the pinion gears. Often times the key in the pinion gears (39-9160) get stuck in the shaft and will not catch the gears. If that's the case, I would remove the keys, and poke out the small springs under the keys and clean, grease, insert new springs & keys and pinion gears if necessary. (make note of how you removed those gears and replace the same way)
