You say it won’t move, but you don’t tell if the engine is running or not. Could make a difference. I’ll assume that the engine is running. You have three things to check. One, is the brake releasing, check to see that the belt is tight and the brake pads are releasing the brake disc on the side of the tranny. Two, is the tranny float control rod, on the back of the tractor is pushed in. Three, pull the deck and check to see if the belt tensioner is moving it’s full distance, putting tension on the belt. Finally, I should add that the belt should be in good condition, when was it changed last?