If you power on your computer and the screen remains blank, you may have a video problem. If the POST (Power on Self Test) determines that you have a video issue it may issue beeps. Listen to the beeps and refer to the BIOS documentation to reference what the beeps mean.
You may need to change out a bad video card. If there are multiple video options on your system, you may want to check the BIOS configuration to make sure none have been accidentally disabled.