Taking the above posts into account, a variety of things could be wrong with your computer that you aren't seeing any video. If your hard disk doesn't show any activity beyond its initial spin-up (and probably spin-down if you're leaving the computer on long enough), you might have a GPU issue that's preventing your system from going past POST. PCs from the past few years or so may have the GPU as part of the main processor (examples include the AMD E/A Series, and the Intel Core i-Series), however with the age of your system, I'm going to assume that, if it has onboard video, it probably is controlled on a northbridge chip on the motherboard.
If there is hard disk activity beyond spin-up, such as what you would hear when the operating system is booting, then it's more likely your display or its connection is bad.