I've had this happen recently in both my camper and my '91 Golf.  Most likely it is your solenoid, however the next time it happens, here are some things to check:

Run a cable (like a booster cable) directly from the starter (clip it on to the bottom bolt) to the Negative terminal of the battery, or at least to where the negative cable attaches to the body.  Try turning your key, if it starts now your problem may be a bad ground.

Try moving your gear leaver (if automatic) around a bit, put it in neutral and try starting.  Wiggle it around in park while trying the starter.

The other problem could be with your ignition switch. On the golf I unplugged my ignition switch and plugged the wires into a spare one that I knew was working. Still didn't start, so i know my switch is fine now.

Anyway, I have a feeling that you will see this problem reoccur, so here's some things to try when that happens.

Cheers,
Bob Norman
'81 Westy
Gander, Newfoundland
Canada