Have you connected the PCI-E x16 power connectors from the Power Supply Unit to the Graphics Card?
Did you then plug your monitor in to the Graphics Car4d and not the Motherboard's Integrated Graphics Chipset Video Port?
Have you installed the correct drivers for the Graphics Card? Get the latest one from Nvidia or AMD's website (depending on if it's an Nvidia Geforce-type card or an AMD Radeon-Type card).