Firstly make sure you have seated the graphics card properly, pushed all the way into the PCIe x16 slot.
Next check the PCI-E cable from the PSU is properly connected to the GPU
You need to check if your CPU has integrated graphics. Make sure this feature is disabled in the BIOS (some PCs require manually disabling it)
You could try uninstalling all the drivers and then reinstalling, reboot and see if it works. If this GPU is replacing an old one, make sure all the drivers for the old GPU are uninstalled