My John Deere riding lawn mower worked fine, It got a flat tire so I removed the tire and fixed it. I then replaced the tire. The lawn mower will turn on, the blades will engage, but the transmission won't engage. I checked the rod in the back of the lawn mower and it was in. I pulled it out and pushed it back in and it still won't engage. I checked the first pully under the engine and the one on top of the transmission and they both engage. Does any one know what could be the problem??

There's a square key stock that goes on the axle and keeps the wheel from turning ,if you raise up the side you took off you will see it spins freely , and if you look at the axle you will see were it goes.