Toro Model 57360 Won't Move?

I have Toro Model 57360. When mowing, I heard a clicking noise coming from under the seat (gear box). It then lurched a little and stopped. It tries to engage, but now won't move. It rolls freely in neutral, but not while in gear. When I reach up under the mower, I can't move the belt to the gear box. To let you know how bad a mechanic I am, it took me 13 hours to change the radiator in my car. So I am a real novice. Don't have a manual and don't even know how to access the gear box except from beneath the mower.....HELP!