I am struck by how much white debris is inside that bundle, implying it has been getting wet and exposed to contaminates. You didn't say, but I assume there was one wire that broke. But the copper looks pretty good, so I would suspect it was mechanical failure (vibration, tugging, twisting, rubbing) there and not corroded-through. Could have been weakened due to corrosion and then broke when you manipulated it.
In any case, if you find something like that, it raises the odds that there is yet another issue, somewhere.
I can't speak to the non-starting and then going completely dead again. First part could be so many things, especially if it hasn't run in a long time (presumably at this point, coming close to a year now). Could also be the battery voltage dropped below acceptable. Second part could be that once the wiring relaxed again, another wiring issue asserted itself. Might also need to check battery voltage under load, and all the fuses yet again