Had it been the plugs, the car would have cranked, as the starter motor has nothing to do with the plugs. The one thing that argues against the battery is that the lights worked. Those other circuits draw minimal power, but headlights draw a fair amount. However, if this was during the day, the lights may have been very dim and you wouldn't have noticed.

I would also vote for just replacing the battery, especially if it is several years old. Even if you can charge it back up, you have been given warning, and it will never get better, only worse. You might go for a couple years without this happening again, or it could start being a regular occurence. As an interim measure, jumpstarting the car should get you going on any day when it doesn't start.