Do you have the Windows 7 install disc?
They have a Restore Mode if put it back in your optical drive. Just make sure to back up some current stuff (i always just back up the important stuff, tv/movies and music is all replaceable, plus it takes for'f**kin'ever to back up).
You can also run the Restore Mode (windows button>all programs>accessories>system tools>system restore), take it back to when you to when you think it was working the best, then this should sort out the problem, provided its just a software issue, and not a hard ware issue.
Also, could you post the monitor brand and make? I might be able to find out if other people have had the same problem.
Oh, and i think someone might have mentioned it up above, but did you recently, or maybe some one else, change it to a different display setting? Ive had a monitor work in a higher res while on, but when restarting it, it didnt want to be compatible, therefore, it would act like nothing was going to it, and it just went into power save mode.o
Another thing could have been a windows update, and it updated and gave you a wrong new display driver (yeah, windows has done it before). You can try to update your display driver manually. Click on the Windows Start button, then in the text field (Search Programs and Files), type "Device Manager", minus the quotes, then open the device manager, then look for the the Display Adapers; now right-click it and update the driver.
sorry all this info is all jumbled up, why i am terrible at teaching people things lol
Hope this helps.
/nerd