Apple is also aware of this problem and has issued a support document for this situation. In this document they summarized what every nerd might already know but will help every normal user who is in the situation for the first time that the Mac does not start after an update. Apple suggests the following steps:
- Hold the Power Button for 10 seconds. If the Mac was turned on, it will be off after that.
- Remove all connected devices and also SD cards. Switch the Mac back on
- If the problem persists do a SMC reset.
- If the problem still persists reset NVRAM or PRAM.
- IF nothing works contact Apple.
Apple writes explicitly that the support page will be updated as soon as more information is available. So it seems that even the problem is not yet fully understood. If you read point five of the above list, you can at least assume that Apple might be willing to be fair to a damaged device. But: if the update goes wrong, you will not have a device for two weeks – so think about the update carefully.
Apple has released a new installer from Big Sur, but without changing the version number. However, it is not clear if this also addresses the mentioned problem. As soon as there are final statements here we will let you know.