Chances are of what happened is someone ran something over that was hard and sheared the flywheel key. So now the Ignition timming is off. So when you try and start it, the ignition is firing too late. Pull the top cover off, remove the flywheel and replace the flywheel key, replace everything and you should be fine as long as the crankshaft isn't bent underneith the mower.
David, July 2007