After listening to countless owners ask why their mower won't start over the last fifteen years these are the most common problems.
Bad Fuel, bad fuel, bad fuel, plugged carb from bad fuel, plugged carb from bad fuel, plugged carb from bad fuel.
Safety switches not engaged, not in neutral, clutch not depressed, under seat switch loose plug, deck switch left engaged.
Corroded battery connections, bad battery, battery discharged, starter solenoid failed.
Dies after 20-30 minutes. Plugged vent in the fuel cap. (Cub Cadet/Craftsman/MTD tractors are famous for this)
90% of the time it's not a broken part - it's simply an operator error.