Do I understand correctly that you installed a new video card and now you don’t see the imac internal display? Check if there is an image on the external display. I have a problem installing 780m in imac 27 2011, the internal display does not work. The external display shows OS 10.15.4 and 10.13.6 and boot screens. as if imac had turned into a mac mini. @Ausdauersportler and @highvoltage12v tried to help me with AppleGraphicsControl.kext, but no results yetIt’s high sierra, but there isn’t anything on the screen. And it just reboots and makes the sound..
although it does not sound too logical when I put it like that)..