Um, you cannot just disable a DX version, the software you run dictates which version is needed. XIV requires DX11 (or 9c) and your PC (GPU) -supports- DX12 amongst others (6, 7, 8, 9, 10, 11, 12 and their point releases) and you cannot just force a version to work on software that does not support it. You're going to have to do a little legwork to get things going again. Seeing as I've never had anyone try to do this I'm not able to advise you on specific steps to take, it might be a simple deletion of a config file, or a Windows reset/reinstall. It will depend on how you have done this. Best advice I can give you for the future is that unless you know exactly what you are doing, do not alter Windows settings in such a manner.