Does the GPU`s fan spin? If not it could be that it's not getting power, so try and reseat the graphics card and make sure that the power cables are plugged all the way in on both the GPU and power supply side. If the fan does spin double-check that the cable to your monitor is plugged into your GPU, not the motherboard. I can`t tell if you mean it doesn't display anything or if you can get your computer to turn on and use integrated graphics. if The computer doesn't display anything and because you replaced the CPU there could be a change that your motherboard needs a Bios update to recognize the new CPU depending on how new it is compared to your motherboard.