It seems that some older Macs have crashed with a black screen when trying to update to the new one macOS Monterey. Once the new operating system is installed, they no longer start, being totally useless.
A year ago something similar happened with Big Sur macOS, which was blocking some older drives in MacBook Pro laptops. Definitely a serious problem, since you are left unable to use your Mac. Hopefully Apple will fix it quickly.
As a significant number of users are reporting on social media and specialized forums, some older Macs are they have blocked after upgrading to macOS Monterey.
All these user reports suggest that the problem is affecting some older models of MacBook Pro, Mac mini e iMac. The most current models, such as the new generation of Apple Silicon, apparently are not having such problems, since there are no complaints from their users.
https://twitter.com/nj10_Akhil/status/1454286887233802240
This is not a new problem for Apple. Last year, with the launch of macOS Big Sur, something similar already happened. Similar complaints appeared from some MacBook Pro users who saw their laptops freeze and unable to boot after upgrading to Big Sur macOS.
Apple is currently testing an update to macOS 12.1, but this is not expected to be released to all users for a few weeks. macOS Monterrey is still in its first version, and the best thing that users suffering from this error can do is wait until a new macOS Monterey update is released before reinstalling macOS Big Sur.
Surely in Cupertino they are already working around the clock to solve said problem. It is not acceptable that you try to update macOS to a new version that according to the manufacturer is compatible with your Mac, even if it is old, and is totally unusable. Apple will fix it for sure.