There are many reasons why this may be happening. Personally, this has happened to me too and I figured out the issue by the listening to the beep noise that the motherboard makes when it boots. For me, my connections where loose to the PSU from the graphics card, so after tightening up that it worked again.
To figure out your problem, you need to listen to those beep noises it makes. They should be in a series of short and long beeps.
If there is no sound then you might have a loose connection to your graphics card externally, or you are plugging the cable into the motherboard header instead of the graphics card.
But first try to dust everything off and make sure the internal cables are all tight. Also check if the fans spin on the graphics card (if it has any).