Apple has released macOS 12 Monterey, but it is not playing well with some older hardware. For one reason or another, the update has bricked several Intel-based iMacs, MacBook Airs and MacBook Pros. Apple has suggested a workaround, but it does require resetting your machine's firmware.
https://www.notebookcheck.net/macOS-12-Monterey-has-made-some-older-Apple-iMac-MacBook-Air-and-MacBook-Pro-machines-unusable.576837.0.html