This is called a horror update: A new update for macOS can render Macs unusable. It is only with difficulty that the computers can then be restored – because Apple support does not always know what to do either.
macOS Monterey 12.3: Update makes Macs unusable
Mac owners should act with caution with the recently released update to version 12.3 of macOS. In many cases, if the logic board has been replaced, installing the update will result in an unusable computer sometimes doesn’t even boot anymore. Reports on this can be found on various forums, on Reddit and on official Apple support.
In some cases, the update process is said to have completed successfully, but after a reboot, owners expect one instead iBoot Panic message. The device can then no longer be used. In this case, the Mac needs to be put into DFU mode to be able to revive it manually. This in turn requires a second compatible Mac. If you don’t have one, you have to rely on Apple support.
However, as it turns out to be aggravating, Apple’s support sometimes misjudges the problem. Users report that their Mac simply gets a new logic board – and the Update problems start all over againsince an exchanged logic board is currently responsible for the difficulties (source: MacRumors).
In the video: This is what macOS 12 Monterey offers when everything goes well:
Massive update problem: Apple is still silent
Apple has not yet commented on the serious problems. Affected users currently have no choice but to have one if possible second Mac to revive to have nearby. If an Apple Store is visited, it is worth informing employees that the logic board has already been replaced.