I recently was trying to figure out if I could fit another gpu into my motherboard. Just to see how it would look. My computer was working fine before that and now it doesn't want to display anything. all my fans turn on but nothing on the monitor.
Are you running off Windows 10? If so then please replace the GPU in it's initial position, remove CMOS battery for 5 mins before replacing and try and see if the issue is resolved. Can you get into BIOS? Please pass on your full systems specs.
If you're not on an installation of Windows 10 then follow the guide linked below: