Car lost all electrical while driving

Voltage regulator. I've have seen plenty guys keep a few spare regulators in the car just in case. Worth a shot if you want to try for any easy fix, it's a cheap part and takes 3 minutes to replace. If the symptoms come back then you've effectively eliminated that cause.

Had a similar thing happen to me many years ago though mine was completely kaput. Was driving my old W100 down the BQE in NYC one rainy night when all of a sudden at 55mph, the truck literally dies as if the key was suddenly turned off. Managed to get it off the highway down underneath without getting killed but unlike your case, still no start, no nothing. Had it towed to a local shop, guy was old school, knew right away what it was. He was exactly correct.

Yours may be on the way out but perhaps you got lucky this time.