Lower the deck to the lowest setting and lie on the ground and inspect the ground drive belt. This belt goes from the engine pulley, through some idler pulleys, and around the input pulley on the transaxle.
Engage the parking brake and start the engine. See if the belt is turning the input pulley on the transaxle.
Put the tractor in neutral with the parking brake disengaged and see if you can push the tractor by hand. If the wheels won't turn, the brake shoes (pucks) might be stuck to the bake disc on the outside of the transaxle. Get someone to engage and disengage the brake pedal while you watch the brake arm move forward and back. You can probably spray the brake mechanism with WD40 or something similar and free up the brake shoes if they are stuck..