I am also having the same problem since yesterday. Also thinking that my video card is spoilt, I went to buy a new one. But then the problem still remained unsolved. I also thought that the culprit must be my motherboard.
Trying my luck, I replaced the CMOS battery with a new one, and voila my computer sprang to life again. I had been troubled by this since then: Can the problem really be the battery? It sounds so incredible, or is this just a temporary solution?