I've seen some odd problems with a couple of monitors in the last few weeks.
A few weeks ago we had a very quick power surge at work. Not much later, I received a call from a user saying that her computer froze up. I went to take a look, held in the power button to shut down the computer. The computer shut off, but the image on the monitor remained the same. I tried pressing the power button on the monitor, but it would not turn off. Unplugging the power for the monitor and plugging it back in worked and fixed the problem. I never saw a monitor freeze up before!
Today I had a similar problem, except this time the screen was blank. I rebooted the computer. Still nothing. All connections were fine. I could not turn off the monitor. I unplugged it, plugged it back in, and it came back on - with a blank screen, not the usual monitor "boot up". Again, I could not turn off the monitor. Wondering if something was wrong with the video card, I grabbed another monitor and plugged it in - no problems there. While the problem monitor was not connected to the monitor, but was plugged into the power, I decided to unplug it from the power and plug it back in. This time the monitor looked normal. I reconnected it back to the computer and all was fine again. What in the world happened remains a mystery to me.