Try a different screen. Something tells me you have a problem with either the screen or the video card not powering up. Also, did you make sure that the external power from the video card, if any, is plugged in?
Ya monitor is turned on right? lol just kidding. Is the graphics card definately seated properly, the catch should lock in. Are all the power connectors attached? Have ya got another graphics card lying around thet you can connect, to rule out a problem with the new card?