This can be caused by:

1.The monitor is not connected properly.
2. Bad hardware

Verify that the monitor is properly connected in the back of the monitor as well as to the back of the computer.

If the connections are OK it means that monitor or the video card is probably damaged. To test which component is dameged connect a different monitor to your computer or connect your monitor to a different computer.

If the other monitor works on your computer it means that your monitor is bad and if your monitor works on another computer it means that you probably have a damaged video card.