I would first pull the plugs and see if it turns over.

If not check to see if you can turn the driveshaft by hand and if if moves smoothly it is likely the starter or the relay. If it doesn't move easily check the impeller to make sure everything is clear and then possibly spray carb cleaner into the cylinders and let it sit for half hour to evaporate. Then right by the connection between the impeller shaft and the engine there is a hole in the driveshaft you can put a screwdriver into it and try to manually turn the engine this way. If it breaks loose try to turn the starter again and if it turns then put it back together and give starting it another try.