I was installing a used (but working) video card in my son's custom computer (that I built so I know it is good), flushed Bios before installing, and still cannot get the computer to recognize the card and have video. I took the card out (flushed bios again), and the mother board video outlet doesn't allow video now either.... got me stumped.
I would turn on the computer with the monitor cable plugged into your onboard video. I would then go into the bios and see what the video adapter setting is set to whether its looking at the onboard video first or not. If so I would change it to pci express save your settings and exit. Then install the card and boot up.