In a statement shared by Apple and posted by Rene Ritchie on Twitter, “We have identified and fixed an issue with the firmware on the Apple T2 security chip that prevented a very small number of users from booting up their Mac after updating macOS. The updated firmware is now included with the existing macOS updates. Any users impacted by this issue can contact Apple Support for assistance.”
Keep in mind that this fix does not fix Macs that were bricked as a result of the update, but rather it fixes the issue so that Macs that haven’t installed macOS Monterey yet will not run into the same problem when they do install it. For those who have been affected and have a bricked Mac, you’ll have to go to Apple for further assistance.
So far it appears that the affected Macs are all Intel-based Macs, so if you’re using one of Apple’s newer M1 Macs, this (hopefully) shouldn’t be a problem.