This season started well... until the mower died while I was enjoying the first cut. It will not restart. When I turn the key the engine does not crank, or turn or whatever, it simply makes a humming sound. The humming sound seems to be coming from under the steering wheel near the battery. We had the battery tested and were told it is fine. We replaced the battery connectors with new ones in case they were bad. I've checked the safety switch under the seat (unhooked them, cleaned them off, checked for breaks or shorts in the wires, and plugged them back in) but I don't know where the other safety switches are or how to check them. I'm stumped as it was running, died and won't restart. Any suggestions?

When you say it makes a humming sound when you turn the switch that makes me think that the safety switches are OK. If they were keeping it from working it wouldn't do anything. It sounds like you might have a solenoid or starter problem. This wouldn't have made it stop while you were mowing though. That is what is confusing me.

Well one of the problems appears to be the bendix on the starter motor. They are designed to break easy as a safety device.

Take off the blower housing and watch the starter gear. By turning it on, it should engage the starter.

There may be a chance that pieces of this jammed and the flywheel key sheared. Don't know. One problem at a time.