Seasonal PSA: If your gas-powered mower won't start this spring, you probably just need to put fresh gasoline in it.  If the tank was mostly full and sitting all winter, you should empty it out first.  If it was mostly empty, you can just add new gas.

If it still won't start, buy some "carb cleaner", take off the mower's air filter, and spray some into the hole.  Pull the starter until it tries to start (it will probably die immediately).  Repeat the spray+pull cycle several times.  Eventually the motor should start and stay running.  What you've done is forced the motor to suck the remaining stale gas and gunk out of the carb.

Dev, woodworker, St Louis native, small time trader

Keeping this in mind cause I know I’ll have to do it this spring