I had a flat tire and I removed the tire when I put it back on it would not move or go into gear. It would start and the blades would engage but not move. I took both tires off and one side would spin but not the other when in gear and of off the ground.

On the motor shaft under the motor there is a belt just above the belt that goes to your blades. This belt goes back and goes around a pulley on your transmission. It is either off, broke or so loose it will not turn your transmission pulley. Replace belt. And mayby pulleys.

P.S. It might be the idler pully also.