You mean the engine doesn't turn over?
Check your main battery cable connections and also
the earth leads. Otherwise, the starter could be jammed in mesh with the flywheel ring gear-try rocking the tractor back and forward a little with high gear engaged or use a spanner on the crank bolt.
If the m/c has stood for a long time the piston rings could be rusted to the bores. Again, try tuning the engine by hand.
Mr. Toad, September 2010