iMac not turning on after power cut mid update
You covered Apple’s triage steps nicely and I think you hit them all.


https://support.apple.com/en-us/HT204267


Unless you can see something on an external monitor or the internal monitor booting with an option key held or internet recovery and see signs of life on screen, your next step is a repair shop to determine which component inside has failed.


https://support.apple.com/en-us/HT204156


My hope is you can boot to internet recovery and choose to reinstall the OS. Since you were mid update, it likely left your data alone and the system totally needing a reinstall. If you weren’t updating, the hardware failure is more likely, but can’t be ruled out based on the symptoms.

You can also bring a bootable USB to that Mac and test operation / hardware without needing service help.