Hi Jack.
From what you describe, it doesn't sound like the switch to me.
If you press the switch to off, does the one that lit, go out? Come back on?
IF so, it's not the switch....it's something else.
Sounds like maybe a short or a bad ground somewhere.
I would start looking at the connections at the lights themselves and make sure everything is right and tight.
IF they were working when you headed out, maybe something vibrated loose?