I had this problem for a long time in my Windows XP machine. I had set up my monitor to turn off after 3 minutes of inactivity for power saving reasons. I, also, had disabled the screensaver although the last time it was enabled it was set to appear after 1 minute of inactivity. This piece of information will become significant latter on.
The problem was that sometimes my monitor could not go into power saving mode. I had absolutely no idea why such a thing happened and I couldn’t correlate this problem with any user action I was performing each time the problem arose. I knew that I could fix this problem by just clicking Apply in the Power Options control panel applet and I, also, had figured out that the problem disappeared if I adjusted the inactivity period down to 1 minute.
If you don’t want to go into all the technical details that will follow, you may want to know that the cause for this problem is the Adobe Flash player. The problem happens after viewing a Flash video (e.g. YouTube) and there is a bug report for this issue: FP-5216. If you want to learn more please continue reading and/or watch the screen-cast: Part 1 and Part 2.