There are only two things that kill the power to the whole vehicle.
One is a bad battery cable (positive OR negative), and the other is a fault with the Black with yellow wire, coming from the starter relay/solenoid.
Either at the relay, or up under the dash at the back of the ammeter.
Since the positive battery cable is connected directly to the black wire because they share a single post on the starter relay, there’s no component anywhere else in the system that can break that bond.
Only the wires themselves.
Check that the negative battery cable has not come off, or loose, or failed from the harsh environment underneath.
The black wire has a fusible link that can possibly fail (rare, but not impossible), or the ring terminal at the end has failed, or maybe you were working under the dash and disconnected the black with yellow stripe wire there?
The most common failure point of this is a battery cable.
They fail internally, even though they look perfectly fine, or they have simply aged out. I had one that was only two years old and looked brand new that gave me intermittent fits.
Several others here have experienced the same issue. Oddly enough, it’s almost always the positive battery cable.
But don’t overlook your negative battery cable. First thing to do on that whole list would be get there and check that the negative battery cable has not come loose from the engine block.
Not because it’s the most likely problem, but because it’s possible, and it’s one of the easiest things to check.
