Find which pci-e slot and which video card you know work together and show up in windows.  Then work through different combinations of each riser/pci-e card/gpu with the one known good pci-e slot one at a time to rule out which parts may be bad. then work your way through the same way through each mbd pci-e slot with just one known good combination of riser/pci-e card/gpu to make sure teh mbd is good.  if everything is good individually then rebuild the os from scratch.  i like to get spare ssds since they are dirt ass cheap now to just stand up a new windows install and have the old drive as a backup.