...old 27" iMac (2019) that came with the slowest, worst hard drive you can imagine for a modern computer, and which was "bricked" by a Big Sur update, but has been since upgraded to a 2TB SSD, I can nearly guarantee these old Macs are not in fact bricked, but are just taking many, many hours to complete the upgrade. They won't turn on, because they're still running. The confusing black screen can sit like that for several hours (maybe even 12+) while the clunky old drive chugs away quietly as the owner pulls their hair out and thinks their machine has died. I would advise them to just let do its thing for a few days, but that of course would be ridiculous. They will in all likelihood have to put in a support call, go through all the scripted troubleshooting procedures with the help desk tech, then have them book an appointment at certified repair shop and be prepared to pay 4 or $500 or to get the thing fixed (and the drive replaced with an SSD). On the bright side, their clunky old HD-bound Mac with feet of clay will then seem like a whole new machine.