What card did you have before you changed? An ATI card? If so then use Driversweeper to remove all ATI drivers from your computer.
When you plug in the new card you should have your monitor plugged into the card and not the integrated graphics. If you’re not getting a signal then double check/re-plug and connect all power connectors and re-seat the card to ensure a good connection. If that does not work then maybe you did get a defective card.