Hi there,
Without knowing the details of your PC it is really hard to say why the Graphics Card isn't working. It could be due to insufficient power from the PSU, a faulty graphic card or many other things.
If the on-board graphics are working OK the first thing to do is check that you are running the latest BIOS, if you are not, update using the QFlash utility and see if that helps. If you are running the latest BIOS could you post back with your full systems specs as this will help with further troubleshooting.