I'm not 100% sure but I believe these tractors have a two stage clutch where you push half ways down and the transmission is disengaged and when you push the pedal all the way down the pto is disengaged. What I think has happened is that the pto clutch is not releasing and also the pto shaft and the transmission shafts are seized together.
Let me explain a bit more about a two stage clutch. There are two clutches on the flywheel. One for the engine and one for the transmission. These two clutches operate two different shafts and the one shaft is inside the other. Ie one shaft is hollow.