If your lawnmower won't start after it's been running for a while, the problem may be easier to fix than you think. Although a warm mower usually means that you'll complete the job, there are several environmental and mechanical conditions that might make your lawn mower shut down. Some remedies have already been included in most manuals, but others require a closer look, and as always, use the proper safety precautions before troubleshooting any problems.
Priming the Engine
This is always the first solution for an engine that won't start, and most mowers should have a primer button. Primers move air in to the carburetor, which forces gas down through the jet. If your primer isn't doing the job and you have a carburetor with a diaphragm, the diaphragm may have a tear in it. With float carburetors, air can leak around a bowl gasket that is not sealed properly and fail to push gas through the jet.
Bad Spark Plugs
If your lawnmower won't start up again, your spark plugs may have given out on you. Some people disconnect their spark plugs before putting their mowers away for the off season and fail to reconnect them properly, which means a failed initial start. But, if the sparks gap, or space that houses the spark itself closes up, you need to adjust it. If you do get a spark, in older machines, it might mean getting new points, in newer machines, an ignition module could be the problem.
Your battery may have lost enough charge to keep your mower running, so try recharging it and starting it again. If it does start, let it run for a while to build up power. Another problem might be corrosion around your connections, if you have a white powdery substance on your ports, clean them with an old toothbrush. If your battery is good, then the connections might simply be loose, so disconnect and re-tighten them. If none of these suggestions work, it's time for a new battery.
It's important to remove any obstructions in your yard, like large rocks, that can cause damage to the blades. Bent blades mean damage to your crankshaft, which will fail to produce enough inertia to start your engine. You should also look at the blade lock key, if this is broken, the blades will turn too loosely and the engine will "fake" start, but won't turn over. If you think it may be your blades, disconnect the spark plugs and then drain out gas before turning your mower over.