Why did my mower just stop moving?

I have a mid to late 90s model Craftsman riding mower, 19.5hp twin II briggs, 42" cut. It has the automatic transmission, and I was mowing and mower just stopped moving, i turned it off and looked underneath and the drive belt was off, so i put it back on and the mower still would not move. I bought a new belt, (AYP brand) installed and mower still will not move. I've checked tension on belt and its tightening when the clutch brake pedal is released, checked the keyways to make sure both wheels have them, checked the bypass valve to make sure it was pushed in to the "mow" position, not the "push" position, and don't know what else to try. The rearend was not making any noise when it quit pulling, and still not making any noise. Is the rearend just gone and need replacement or something else i cai can do? I know its an old mower, but was running great until it stopped pulling. Thank you for any information