Is the mower out of gas?
Did you hit, or run over something with the mower in the grass?
Oh yeah, people do run out of gas, put their mower away in the garage or garden shed with an empty fuel tank and forget about it until next time. Only to pull on the starter rope until they either break the recoil starter rope, or further frustrate and exhaust themselves. Do yourself a favor, check the fuel tank and make sure it is not empty.
Hitting something in the grass will cause mechanical problems in a lawn mower engine. Something as seemingly harmless as hitting a baseball in the grass, exposed tree root or some other foreign object will do several bad things that will in turn. Kill the engine straight away or keep it from starting after shutting the mower off, damage the blade, bend the crankshaft or shear the flywheel-key all of which are expensive to have repaired at a mower repair shop. Many repair shops will tell you, buying a new mower is a better investment.
The most common thing that happens in cases like these, when the mower blade hits a small foreign object or tree root the flywheel key, by the way an inexpensive part, gets either partially or completely sheared. Throwing the engine ignition out of time the mower may stop running, backfire, run poorly or not start after shutting the engine off.
Tip, when the starter rope handle jerks back, the engine is most likely, either out of time, or the engine cylinder is flooded with raw gas or oil from the crankcase, remove the spark plug and pull the rope to clear the gas and oil from the cylinder.
The fix, First remove the spark plug from the engine, with the plug removed, place the mower on its side with the carburetor / air-cleaner cover up. Then check the mower blade for damages, is it bent, chipped or has gouges in the cutting edges.
Rotate the mower blade in a clockwise rotation, while watching the engine crankshaft. You are looking to see if the crankshaft wobbles side to side while it spins. This is to ensure the crankshaft is not bent. When it wobbles from side to side the shaft is bent, and it is time you buy a new mower.
Otherwise, replace the damaged flywheel key and you will be back in business. With the mower back on its wheels, it is time to fix the damaged flywheel key. Remove the blower-housing, top covering the flywheel. Pull the flywheel off the top of the crankshaft and replace the sheared flywheel key. Sounds easy right, it really is but here again. Each engine has its own specific steps where a good Haynes small engine repair manual helps guide you through the simple step-by-step procedures.