The unstately Wagner manor was getting gawdawful hot. I walked down the master hallway (when you only have one, it gets to be the master hall) to the thermostat and pushed the button to lower the temperature setting. Nothing happened. The air conditioner remained silent. But the digital display flashed on and off. Something was up. Sighing at yet another gadget to fix, I removed the front plate to get the model number. Then I was off to the computer to download a copy of the manual. Minutes later, I was back at the thermostat with the manual in hand. I tried to enter the programming sequence, but the device stubbornly refused to allow any changes. Right about then, I noticed that the master switch was set to the oh-eff-eff position. Yes, it was off. The air conditioner works now.
...and yes, it's true that I'm a professional electronics technician. Why did you ask?