Power Save Mode

Question about a problem I had this evening. Dell flat screen. I had the computer up and running, left it a while, an came back to it. I do that all of the time without incident. This time I bumped very slight (don't know if that was the cause) while plugging something into a usb port and the monitor suddenly showed a black screen that said (something like) "Power Save Mode. Press any key or move mouse to continue." Nothing I did would remove this message. I checked the mouse and keyboard cords in case they were loose, and they weren't. Also checked the monitor cords. I tried all three buttons (power sign, -, +) on the front of the monitor. Also tried powering off the computer. In desperation I unplugged the monitor and the computer and that fixed it.

Questions: How did I enter this power safe mode? It looks like a monitor function. As far as I can tell, I did nothing to cause it. And how do I get out of it once it starts? This has never happened before and was not caused by leaving the computer standing, because I do that all of the time. Any ideas appreciated.