So that's a carp shot if it's any good.
At least check you connected the PCIe power or such to the GPU as that's the most common gaffe then test with another GPU. Try what you have or what you can borrow.
Be sure the HP's BIOS is their latest since it can matter.
I have a P7-1414 HP that I have installed EVGA 600 watt and my Nvidia Geforce GTX 590 in. I plugged the VGI to the graphic card but no signal when startup. I plugged it back to the motherboard VGI and got signal. I have verified BIOS that the integrated graphic is deactivated but I can not get the computer to use the Nvidia card. It shows it in the Device manager.

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