remove the engine cover, (trust me, you see it, it's what covers the entire top of the engine, also known as a blower housing) locate the ignition coil by following the plug wire to it... there should be a small kill wire hooked to that coil. remove it and try to start the mower, if it starts, the kill wire is shorted to ground or there is an issue with the keyswitch grounding the kill wire. IF it does not spark with the wire unplugged, bad coil.
assuming it has a new spark plug.