Ensure that you have the two 6-pin PCI Express supplementary power connectors connected from the power supply unit to the two power sockets on top of the graphics card. If that has been done and it now works you're set to go.
If you still don't get a video signal from the graphics card you will need to go into the BIOS Setup utility and check the settings.
BIOS Setup --> Advanced --> Chipset --> Internal Graphics
Refer to your motherboard manual for info on the various settings and options.