Any ideas why I have zero power? After starting my 84 El Camino yesterday,as it typically does when cold,it stopped, so I put it back in park to restart it like usual(usually takes 2 trys), & everything is dead. I mean everything. No interior lights, or headlights either

 

May guess corroded battery cable.

 

my '68 el camino did this when the floor mounted hi beams switch failed. My wife was driving, said if she tried to go from hi beams to lows the motor died and lost all electrical. Would only start and run with high beams on. Knew she had to be crazy - but I tried it and she was right! She and that car never got along! Probably not your problem but an interesting reason for a car to totally die.

 

I have had batteries drop an internal connection and go totally dead that quick. Grab a volt meter and start at the battery. If the battery has voltage check for the previously mentioned corroded cable. If the cables are not corroded it could be a bad cable to end connection. Basically just follow the path the electrons will take until you find the break in the road