I have this problem many times most specially to my costumers, this is what i do(video card is the problem)
first of all check all hardware and connection to the motherboard if is running properly.
2nd UN-slot video card adapter clean the agp/pci slot and video card with paint brush take off all dust and cobweb on it,
3rd get a pencil eraser and the video card, rub the eraser to the gold part of video card do it side by side until its brighten/shine like gold.
4th slot again the video card to the mother board,check the other parts connection turn it on if all are ready,,
pm.. if what the result to you,,, thanks..