Westwood T1200 ride on mower?

I have a Westwood T1200 ride on mower- recently I started it and ran up and down the garage fine. Next day found I had a puncture on back tyre which I proceeded to take off I removed bolt but could not remove wheel - so I used a hammer and some 3"x2" wood on the inside of wheel and knocked it off. I had tyre repaired. When I started engine and put into gear tractor would not move. I jacked up rear wheels started up, put tractor into gear and back wheels moved very slowly. I checked all I could but did not get tractor moving again.