The video card is bad. The cable is disconnected. The monitor that is plugged in, cannot support the video settings that are applied to it. The monitor isn't plugged in. The monitor doesn't have any power applied. The monitor isn't turned on.
All of these are valid reasons why it isn't working....get it?
As you can see, there are many causes to the issue you are describing. Without giving any personal info, please describe what no display means!
If you were to press the On screen menu button on the monitor, what happens? Does another menu come up? If so, the Video card is DEAD, or the cable is unplugged!