Murray Lawn tractor wont go into gear

It started when I went to cut the grass. I put it into gear and started to roll, and low and behold one of the tires was flat .

I put the rear of the tractor on a railroad tye to hold it up while I removed the tire. I then brought the tire to have a tube put in it.The next day I picked up the tire and brought it home and put it back on the tractor and removed the railroad tye. I then started it up and put it into gear forwards nothing rear nothing... What gives ?

Thanks

Ed