Something is telling me this is an OS/driver issue.
Does this happen when you put the old graphics card back in or did you even change the graphics? Please list what parts you changed other than the case.
If you did change the graphics card.....
First thing I would try is the drivers. When you get into the OS use DDU to remove them then reinstall. This is first rule when changing graphics cards always. When I do it I uninstall the drivers before I change the cards. Then boot up with the new cards and install the drivers.
That’s pretty much outside of the limit for the UEFI.*
Of the graphics driver scrub and reload doesn’t work it’s looking like the easiest out is to do a clean install and start over. *Can you get into the BIOS?
Have you tried a different PCIe slot?
Bad drivers may keep you from seeing anything after the initial post of you had fast boot enabled. *