John Deere riding mower won't go in forward or reverse. This happen after I jacked it up and changed a flat. Any ideas? Please

Mine has a thin metal rod sticking out the back, with a 2" bend at the end, for releasing the transmission. Turn that and see if it goes back in.
Look for it in your manual.

Hi,
Rear axles are keyed between the axle and the wheel. The key is missing. Hint: If you watch that side with someone else aboard the mower, you will see the axle spin within the wheel in the side in question.
The key is 5/32" x 2"
Peace,
Clarke