Hi!
Hopefully I can help you further! You mentioned that also the AV cable failed to show you any image, right?
It’s possible that the firmware update failed. This does not mean that the actual OS installation failed but failed on the “BIOS/Firmware” side. This could be caused by a harddisk which failed..
If this is the case, it is bricked and it’s motherboard related. Repairing this is very difficult, do you have any experience with soldering? Replacing the motherboard is possible but expensive.
But, before we jump to this, what does work at the moment? Are you able to access safe mode successfully? Till how far can you go before you get a black screen?