Read the spark plug first to try to see what has been going on. A plug in normal condition has a thin layer of light-brown deposits on it. If you see a thick layer of black carbon deposits, it's a sign that the carburetor mixture is too rich -- your engine may even be smoking. Electrodes that have been worn thin are a sign of overheating, often caused by too much ethanol in the gas. If your plug has wet, oily deposits, it's time to take the mower in for servicing, because oil is leaking into the combustion chamber where it shouldn't be. · Other Engine Problems · If your spark plug appears to have normal wear -- or if you've just installed a new one -- and the engine doesn't fire, you can check the plug by replacing the spark plug wire with a tester and pulling the starting rope. If you don't see a spark on the tester, there may be a problem with the ignition coil of the flywheel to which it's attached. The ignition coil doesn't frequently develop problems, but the connection between the flywheel and coil or between the starting pulley and flywheel can wear out, and the coil may not be turning. Replacing the worn part isn't difficult, but you may need a pro to properly disassemble the engine. Answer from mayer on ifixit.com