When a gas engine doesn't run it is either because you have no fuel or no spark. It is easy to verify if you have spark, as Whiskeydan suggested. Once you verify spark, then start tracing back along the fuel delivery. If you have really old gas in there and didn't use fuel stabilizer, you might have gummed up the carb or fuel line at some point.

One other thing to consider, and I don't know if your lawnmower has one, but if you have a fuel shut off valve and it somehow got nudged closed, then you won't get fuel going into the carb. I have a valve like that on my snowthrower, and I tried starting it once after forgetting I had closed the valve.

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Jonathan

2011 Mazda2
2000 E320 4Matic Wagon
1994 C280 (retired)