You said nothing about the system you are using, so it is difficult to say whether it is a system issue or a monitor issue, need more information.
I have one LCD Monitor SAMSUNG Syncmatser 923 NW ( 19") that had its resolution changed and from than it turns on and immediately turns black. If I turn off / on it shows the screen and immediately gets back to black / no image.
What is the best resolution to this LCD monitor and how many bits and hertz ? Your information and guidance will be appreciated