We need more info then just it wont start... What were you doing on the tractor? Did you shut it off then wont restart or did it bog down and cut off and wont restart... we need details.
My MX5100 did it, and it was the safety switch for the clutch. Try jumping the wires on the starter to see if it starts. If so it is one of the safety switches. My dealer picked it up and fixed it under warranty.