Mower won't start: the 10-minute fuel-spark-compression check

Fuel: pull the plug, is it wet? Dry plug = no fuel. Spray a one-second shot of starting fluid or carb cleaner into the intake, pull. If it fires and dies, it is a fuel problem: 90% a gummed carb from last year's gas.

Spark: plug out, boot on, plug body grounded to the head, pull the cord in the dark. Blue snap = good. Nothing = coil, kill wire, or the safety bail switch. Check the bail switch first.

Compression: pull the cord. It should resist firmly and pop back. If it spins freely, sheared flywheel key (did you hit something?) or a stuck valve. If it is impossible to pull, hydrolocked with fuel from a leaking carb float.

In that order, every time. Write down what you found.