Can your PSU handle your card? Is your card compatible with your motherboard?
Follow these steps and try to get it working:
1. Power off and unplug your computer
2. Take out your graphics card
3. Plug your display into your onboard graphics (the graphics on the motherboard)
4. Plug in, power on and boot up. Uninstall any drivers for video cards
5. Install drivers for your video card from the manufacturer's site
6. Power down, unplug, and insert your graphics card. Make sure it's properly installed into your computer. There should be a screw on the metal flap on the side of the case, the slot is properly secured (you should hear a click when its in), power cables are properly inserted.