Montiro:
Were there any changes made just prior to this happening? (For example, did you add a new video card or other expansion card? Did you install a new program or new device drivers?)
Have you tried adjusting the brightness and contrast on the monitor itself? (there should be buttons or dials to allow you to do this).
Have you checked that the data cable and power cable are both connected firmly to the monitor and to the computer?
Do you see the memory test and other information appear on the monitor when you first turn the computer on (before Windows loads), or does the monitor stay black with no white letters or numbers appearing at all?
Does the computer give any beep tones? (normally, it will beep just once if no problems are found, but more than once if there is a problems with a hardware device).
Do you have another monitor that you may try to use with this computer? Or, do you have a friend who has a computer who will let you try your monitor with her computer? (If another monitor works on your computer or your monitor will not work on another computer, then you may have to replace your monitor).