[Dragonoake]

Went to the store, and noticed that the digital readouts in the dash were flickering, but everything else was fine.
When I came back out, other than the door chimes, the heater fan light, and some flickering indicator lights in the dash, the entire electrical system was dead.
Tried the key a couple times, nothing
Checked the battery, ground cables, fuses, and relays to find nothing
On a hunch, I tried turning the key back to accessory ....
The dash lights and radio came on, so I tried the key again, and other than the fact that all the gauges pegged out for a few seconds, everything was normal
What happened?

 

[Bob Lincoln]

Bad ignition switch, most likely.

 

[ImperialCrown]

Welcome to Allpar. By 'entire electrical system was dead', do you mean battery-powered things also? Dome lights, door/key-in chimes and power locks are battery supplied and would be powered all the time, independent of the ignition switch.
The top battery terminals can look fine and dandy from the outside, but hidden crust and scale can build up between the battery post and terminal where the current passes (or doesn't) between the two. The only way to see and clean this is by removing the battery terminal from the battery post and scaping/brushing the contact area (reset your clock). It could still be a poor battery connection and things working the second try may have just been coincidence.
The connectors and wiring underneath the PDC (power distribution center-underhood fuse/relay box) has been known to corrode also.