Remove the graphics card and try to boot into Windows. If Windows will not boot attempt a repair with the graphics card removed. Once you have Windows reliably booting up with the onboard graphics then you can have another go at the GFX.
Look in your BIOS before installing the card, do this by pressing F2 or DEL (depending on your motherboard brand, model, etc) - then set your primary graphics to PCIe (or similar), which represents and external card.
Turn the PC off, install the card, with your monitor connected to the card start up and see if Windows loads. If it doesn't and you have the same issue you either have a faulty graphics card or a faulty motherboard.