I was mowing my lawn yesterday and at the same time, there was a loud pop and my air filter blew off. Now my mower won't start. Help?

Backfired... I would bet good money the woodruff key on the flywheel has sheared and the flywheel has moved on the crankshaft. Easy fix if you have the tools and can get or make another woodruff key. Probably happened during a previous use and it managed to move enough to cause the backfire, and is so far out now it won't run.

Thank you Smoothy. I did read about that as being a possibility. I'll be looking into that.