. The main fuse on these old bikes is in a small black box near the battery. It is a thin metal strip and gets corroded and cracks. Replace the whole thing with an in-line blade fuse holder and 30 amp fuse. But make sure your battery connections are clean and tight also. That gives a lot of problems.
Instantaneous shutoff while riding and no power at all from then on, as indicated by OP, indicates what Swifty indicates: main fuse. Source: me, amateur mechanic - mediocre rider - pro bungler.
Typing indicate one more time, no two more times, to indicate a new forum milestone.
Also the notorious starter button/kill switch assembly. Mine quit after a stoppage while out riding. Symptoms: no headlight & no start. Had to be pushed and bump started in 2nd. gear. Battery would be my 1st. analyzation of the process.