Trouble with craftsman mid-engine rider lawn tractor.2 years old-used for one summer. After using for about 30 minutes I stopped tractor to remove branches from lawn. I attempted to restart tractor after about 5 minutes, but engine would not fire . Spark plug is clean and gas tank is full. Tractor is model 536.270320

A safety switch?

Check the fuel line at the carburetor that comes from the tank too make sure that you have fuel too the carburetor. Check too make sure you have spark. Just because it is a new spark plug does not mean you have spark. I would check at sears or an auto parts store for a spark tester. They should know what you are looking for if you tell them what you are trying to do. This is a start anyway. Before you do anything make sure too check that your air filter is clean and the fuel filter is not stopped up. Hope you find the problem and it does not get too complex.