Try what Ronnie S has said, this will ensure that the starter is OK. I would also check to be sure none of the wire connections are corroded. Start at the battery terminals and also be sure that the connection from ground strap to the tractor is shiny-clean.
   Over the 4th of July I had similar problems and found that the starter had gone bad, but there was power to the starter in my situation.
    Let us know what you find.
                     Cal