I have two light switches in my kitchen adjacent to the entry ways (walk thru kitchen). I use to be able to turn off/on the lights at either switch, however now, both light switches have to be in the off posistion for the lights to be off. As I wrote earlier, it didn't matter what position the switches were to turn them off or on.
Most likely a loose connection has developed in the wiring at either or both switches.
Another possibility is that something failed internally in one of the switches.
Those "push in" or "backstab" wire connections (no screws needed) on switches and receptacles have a high likelihood of problems of this kind after several years.
(One morning on WRKO Radio) The tub's not clean! Let's remodel the bathroom.
(One afternoon on WRKO Radio) It's time for an oil change. I'm going to trade in the car for next year's model.