Are you using the driver disk that came with the graphics card? If so, try downloading the latest drivers from the AMD website.
Other things that come to mind:
Make sure the graphics card is properly inserted in the PCIe slot, with all appropriate power supply connectors inserted.
Make sure the cable from the monitor is in the graphics card not the motherboard.
If you had a previous graphics card or were using integrated graphics previously, try uninstalling the old drivers first.