I would start by checking your oil level to ensure it is not overfull. If it's WAY overfull you have a carb flooding the crankcase issue which needs a carb rebuild to correct. If your oil level is OK, remove the spark plug which releases all compression and see if the engine will turn over. If it does you'll know that there is most likely not any internal damage to the engine. If it turns over fine I would put the spark plug back in and remove the valve cover and make sure nothing has come loose and that the valve gaps are set properly. You didn't hit anything while mowing did you? If so, you should pull the engine shroud and remove the flywheel NUT only and see if the flywheel key is sheared. You can check the flywheel key without removing the flywheel. A sheared key will throw the timing off and make it about impossible to start. Get back with us on what progress you have made.
