Ouch! I suspect you where a bit too quick!
During the OS and update processes your system will often appear off, in truth its just not able to display anything as the systems firmware (EFI) is being updated. So what you’ve now done is corrupted the process and pulled the chair from under the system to boot replacing things!
OK, so the firmware is in never-never land which we need to fix. At this point I would carefully put everything back together making sure the system does not look shop worn so Apple won’t object.
Now its time to visit an Apple Store or authorized service center with your system as being the best course.
So is there anything I can do my self? You’ll need to replace the logic board Apple has altered how EFI is setup on the newer systems and using a custom chip.
I have not been able to get the needed chip as well as the firmware. As such you can’t replace the chip with the needed firmware on these newer iMac’s. Apple has made it much harder to fix a simple issue like this. While still not an easy fix as you need to desolder a chip off the logic board and replace it at least it could be done.