If you are using an on-board video card and you don't have connections for your discreet video card, you will have problems. Plug your xbox/ps3 into your wall, then turn them on without connecting them to a TV. They will sit there and blink the power light at you, but never turn on.
The same thing happens to them, that is happening to your video card. It knows when it has a monitor attached, and until it does it has no reason to turn itself on.
Step 1, connect a monitor. Try that then get back to us.