Official instruction from Apple.
Not so long ago, there were reports on the web about problems with macOS Big Sur on older MacBook Pro models. At that time, users complained that the update had made a brick out of their laptop. Apple has acknowledged the problem and has already explained how to reanimate the devices.
So what to do if your MacBook stops starting after updating:
- Press and hold the laptop’s power button for at least 10 seconds, then release. If your MacBook is on, it will shut down.
- Disconnect all external devices from the laptop, including the monitor and USB accessories, and remove any cards inserted in the SDXC slot. Then turn on your Mac.
- If the problem persists, reset the SMC.
- If the problem persists, reset NVRAM or PRAM.
If none of these steps worked, contact Apple Support for more help. The manufacturer will notify about the emergence of new ways to solve the problem by updating the corresponding article on the site.