I had to do a BIOS reset because of a failed overclock. To do this I had to take out my working graphics card to get to the CMOS battery. Now when I put my graphics card back I get no display at all. The PC works fine if the graphics card is not in (just uses onboard GFX).
First go back into the bios, you will need to set your default graphics card output to be the one in the Pci-e slot, make sure the on board graphics are turned off if you can,save the settings and exit the bios. Don`t forget to plug any extra power connectors into the back of the card if it needs them. and of course make sure the monitor cable is plugged into the right output of the right graphics card.