Trickle charged battery. Turn key and still nothing.

Toro Sand Pro 14, 1990 model won't start. Trickle charged battery, nothing when key turned. Tried by passing batttery with charger, nothing. Sounds like a quiet hum under ignition key.

Could be a failed safety switch, most mowers have at least 3 of them. The one under the seat fails most often but the clutch/brake or the engaged blade swcth also can fail.