I actually did some research looking around at different sites and found that the most likely cause of this is a dead or dying video card.
Several things to check...
-Make sure the slot and prongs are clean. ie no dust or debris lodged into the slots.
-Verify that the video card is seated into the slot firmly.
-Verify if the card requires PSU connections that it has them and that your PSU is outputting the correct power.
-Remove the card completely and try it in another system if you have one. If you dont have another computer then possibly take it to a mom and pops computer store and have them test it.
-Purchase an extremely low end video card (50 bucks at the most) and verify the system boots. If it does then the answer is there.
Every site i read had these various problems in which some fixed and some didnt. Unfortunately, in your case the card may be on its last leg. But to test to make sure its going to take a little finess and patience. In the long run you will learn a great deal from this and ultimately will be able to troubleshoot damn near anything. The key to troubleshooting hardware is to remove all parts and put the computer together peice by peice to narrow down the problem. I would start with the video card and atleast make sure your motherboard is not defective.
Goodluck
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