If the graphics on the motherboard still work, then you have to go into the BIOS and change some settings around so the computer knows to prioritize the graphics cards.
When the computer starts, you hit whatever button takes you to the setup. Then you have to change the setup.
It does know its there because when I start it up with my normal in cou graphics card it tells me to plug in the cable to the graphics card. I also have a VGA to DVI adapter than a VGA to VGA cable to my monitor