Have you installed your card into the X16 slot of your motherboard and tried it that way?
Shut down your computer, make sure the video card is in the PCIeX16 slot, go into your bios and:
In your BIOS make sure your PRIMARY VIDEO CONTROLLER is set to GFX0 - GFX0= primary video controller on a PCIeX16 slot.
GFX0 = Primary video contrller on PCIeX16 slot
GPP = Primary video contrller on PCIeX1 slot
IGFX = onboard display output.
PCI = Primary video controller on PCI slot
See if that works.