The first thing you have to do with the mower turned off is to see if the blades turn the belt on the pulleys. Wear heavy gloves and turn the blade. You should see the belt turning if the blade brake is released. If it turns, good.

Next, start up the mower and turn on the electric clutch. Does the pulley turn? If not, then the first place to check is the electric clutch gap between the plates. It should be around .012 If you can adjust the gap, you're good. If not, have somebody look at it.

Southbend, I am not at all familiar with your particular tractor, so what I say will be generic

First, clean both ends of both your battery cables, then reattach them tightly.. Next, find the connector where two small wires are attached to the PTO (power takeoff) (this is a pulley-type unit that is attached to the end of the shaft that comes out of your engine). Check that the wires are not corroded and are tightly attached.

Sit on the tractor seat and turn the key to the "on" position, but do NOT start the engine. Activate the PTO switch (i.e. engage the mower blades - obviously, they won't turn since the engine is not running). You should hear a distinct CLICK. If not, you've probably got a bad electrical connection somewhere, or an electrical short and you will need a multimeter to check things out.

My best guesses are (in order): battery cables corroded or loose; interruption in the electrical wires to the PTO unit; bad ground between the engine and the tractor frame. There are any number of other things that can cause the problem, including the key switch, seat switch, and the PTO unit itself If you don't have the know-how or tools to pursue these things, then you need a good repair shop.