Well, this might be a real long shot, but the same thing happened to my GC.... turned out to be a broken ground strap, going from the block to the frame. Everything electrical --granted, not much on the GC-- seemed to work, except it just wouldn't crank. Had been running fine just a few hours before that.
I might add that I'd been experiencing starting problems prior to that, for about a year; it always started, but seemed to have more difficulty than when it was new. My guess is that the strands in the ground strap had been breaking, a few at a time, over the 3 years I'd had it (coincidence??), before the last few finally let go.
Might be way too simple, but it sure cured my little (man, that 6180 is huge!) tractor.
A fellow TBNer told me where to look, and he was right on the money. Hope that's all it is!
The culprit:
