I had this happen to me three times - twice it was ok the other cost me £1200.

The first was my power cable to the cpu had come lose - the second was my memory was not in currectly after updating my hard drive.

The third killer one was that my bios chip had gone and need a complete new set up. The whole computer was powering up but all I sgot was 2 beeps and nothing else but blank monitor.

I hope its not that.

Strip the computer and put it back together making sure everything is plugged in correctly just to be sure.

Other idea is - is it your graphic card ??