[neilw20]

Sounds like the flipped switch which you use rarely has become intermittent due to lack of use.
Just toggling the switch was enough to disturb the microscopic layer of whatever that was giving it high resistance. This switch in this application is probably running at a current below the switch manufacturer's minimum current spec. This often happens when switches which are designed for switching some power are only run at the current supplied by logic.

Very often the circuit will have a capacitor across the switch, and turning it off and an again is sufficient to clean the contacts, resulting from the high(er) current pulse afforded by the capacitor.

Things ALWAYS stop working sometime. It maybe years until it does it again, but if you see it again soon, replace the switch.

Ahah. The flood system. Look at the environmental rating for the switch. IP?? -- C&K switch (or clone) not meant to run in juice maybe.