Intermittent no-start on 92 Cabrio

[Doug]

Wondering if you guys could offer some advice on a water-cooled VW. My son's '92 Wolfsburg Cabriolet has about 90,000 on it. About two weeks ago, after he drove it about 30 miles in the prevailing cold weather here in Virginia, he stopped for lunch. After getting back in, it wouldn't start. No sound from the solenoid, he says, no nothing. But the idiot lights came on and other peripheral electricals worked. After sitting about 24 hours, it started and has been doing fine since.
Yes, he has a cell phone. But I'd like to diagnose this before it gets worse and take care of it. Where should I start looking for the electrical gremlin? Thanks.

Doug