I have a 1997 Murray 46" 18 hp riding mower i bought new. model #46258X6A. i had to replace a tire on it recently. so we put it on blocks, changed the tire, engaged the push position lever and rolled it off the blocks. i disengaged the push lever and now my mower will not move. you can hear the transmission trying to operate, but it just seems like it's still "stuck" in neutral. The drive belt is fine. It's not loose or broken. The pulleys for the belt also seem fine. The manual says do not add oil or any fluid to the tranny as it is a self contained unit. But, I'm still stuck with a mower that runs fine, it just will not drive. Any ideas?

Many Murray riding mowers have a keyway between the axle and rim. Did you get it back in place when you reinstalled the wheel? If you are not sure watch the axle while someone is trying to get it to move. If the axle spins and the wheel doesn't move it is not in place.