So why would the entire car lose electrical power? I would've thought that the dome light or something not needing much electrical current would still work t least. Wow.
On my 68 Chevy II Nova, there is one 12 gauge wire going to a terminal in a plug with other wires on the engine side of the bulkhead connector and this is my main and ONLY feed to the electronics past it inside the car. Chances are yours is laid out about the same. On the the mating part of this bulkhead connector bolted to the firewall and between the backside of the fuse block and the firewall is another connector that then get's split with 12V going to different thinks like the ignition switch, the headlight switch, and the fuse block.
When troubleshooting you need to know exactly what is or is not working and then start looking for commonality between those things. For example if just the headlights quit working but the dome light still worked and the car still ran, then the main power into the car from the bulkhead is probably good but the issue with just the headlights going off would be a problem with that circuit. It as well as the ignition and fuse block get power from the same main line into the car but if those are working then the headlight issue is not a problem with an ignition switch for example or a fuse block issue.
Look at things in your house, if the power coming into the house went out due to a connection opening up off of the pole in the street, not even an alarm clock would work but if while at home the fridge quit working but the lights in the room worked, you might have an issue just with that circuit for the fridge and not main power coming into the house.
Keep in mind that when current flows heat also increases and if you had a marginal connection on a bulkhead connector it might be good enough to allow the car to run as it might only require a few amps to power the ignition system but then turn on the headlights and now the amperage jumps another 10A and now that marginal connection heats up more and if this connection is in a plastic housing, the connector may spread apart just enough to stop the flow of power. Now with no power passing through this marginal connection it cools and may reconnect itself poorly once again allowing you do go down the road once again.
Touching back on commonality, if my car were to shut off and the dome light and headlights would not work I would then try the horn. How my car is wired is, if the horn did work then I would be looking at connections past the horn relay under the hood to and through the bulkhead connector and then into the car BUT if the horn didn't work also, then I would be looking for issues between it and the battery positive post.