Did you connect the battery backwards? Or, did you connect it red is positive-black is ground/negative.
Or did you connect it the same as it was before you worked on it?
On most machines, the connections are red-to + terminal on battery, and black to Ground on - post on battery. the place where the black / negative cable bolts to the tractor frame can get rusty which upsets the whole thing! Try a touch of the positive jumper cable to the bolt on the side of the starter, to see if it will turn. If still no turn, then using the black jumper cable, touch it to the side of the starter case. Use the battery in your pickup, or car, rather than the one in your tractor, unless you know the tractor battery is fully charged or has enough power to start it. This test will tell if the battery ground cable at the frame connection needs cleaned up.

There are a few probable causes for this (though it may still require some close inspection of the wires and parts).
As you stated "out of the blue". This could happen in a perfectly normal ignition system if the ignition switch was left in the ON position and were closed at the time. A defective ignition switch passing current through it (when OFF) could do the same......but the POINTS WOULD HAVE TO BE CLOSED also.
Now, if none of the above have occurred, there has to be a short to ground somewhere on the negative side of the coil (a short on the positive side wiring would burn up the wiring between ignition switch and coil, but leave the coil intact, and unpowered)
I would inspect the wire lead that travels from the coil negative terminal to the points. If that wire has rubbed/chafed (or the insulation chewed off by mice) enough to short to ground, it will heat up the coil.
If that wiring inspects OK, the condenser wire lead may be shorted to a ground, or the condenser itself is shorted. I lean more toward the condenser wire being the culprit because a condenser will usually just "go open" instead of shorting (open condenser will cause short points life).
I have seen the condenser wire pull out of the condenser and short to ground which results in the engine not running.

There could also be a problem with deteriorated insulator material in the points wire connection terminal (on the points assembly) that is allowing the coil negative circuit to ground through the body of the points.

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