Transmission is driven by a belt from the bottom of the engine to the top of the transmission. Is the belt in place or is it off one of the pulleys? Located above the belt for the deck. Or the brakes can be locked, making the transmission unable to move the mower! The brake should be a mechanical disc brake just ahead of the right axle, Loosen the caliper mounting bolts to free up the brakes.
Drive belt had slipped of the transmission pulley.