2 Answers

Do you have fresh fuel in it? Does you gas or petrol (depending on where you live) have an additive in it for car emissions? If it does, that additive wreaks havoc with small engines. You may have to have your mower serviced if this is the case.   

Give it a light shot of ether, straight into the carbuerator beneath the air filter. See if that gets it to fire up. A lot of times those small engines need a little shaking up after they've had a spell of not wanting to start. If it does start up, let it run a good 10-15 minutes before allowing to shut off.

The thing of it is, if a small engine isn't starting, alot of times the spark plug is getting fouled with unburned fuel. Ether has a higher evaporation rate, and a lower flash point than gasoline, so it tends to reverse the negative effects. I've taken spark plugs out and sprayed them with ether also, and then lit them on fire to dry them out (make sure you use pliers to hold the plug when you do that.). Then I throw the plug back in and it kicks over.

If a small engine isn't outright broke, or so clogged up that it needs to be tore down, you can usually get it to run. Once you get it to run once, repeat what you did to get it to start the first time. Run it, and let it clean itself out. After awhile, they usually come around. Don't be afraid to include some carbuerator cleaner also. Old fuel really gums things up.