Cables aren't connected properly.
Cable(s) are malfunctioning.
The graphics card isn't seated properly.
The graphics card power isn't connected properly.
The video signal from the graphics card is "out of range" for the monitor.
The monitor's light source is burnt out or shorted.
Point is, please list all of your system specs. Check all the cables. Is this permanent or only in certain situations? If this seems like "duh" to you, please don't feel insulted. You stated absolutely no information to go on, nor anything you've tried so far so I'm here to help.