Unfortunately, having the "No Display" problem isn't a easy one to diagnose. It's like asking "Why won't my car start" in the way that it could be many things causing the issue.
We need more information to try and help you. Please list your system specs and we can go from there.
If you have a GPU installed, remove the card and try using the on-board video and see if you get a display from that. This will at least narrow down if it's your video card that is the problem or not.