Startup in Safe mode. https://support.apple.com/en-gb/HT201262
To leave Safe Mode, choose “Restart” at the bottom.
Quote:
Safe mode prevents your Mac from loading certain software when it starts up, including login items, system extensions not required by macOS,
and fonts not installed by macOS. It also does a basic check of your startup disk, similar to using First Aid in Disk Utility.
And it deletes some system caches, including font caches and the kernel cache,
which will be created again automatically as needed.
If you have tried all suggestions made in this article
If your built-in camera isn’t working on your Mac
and didn't help, please contact Apple. contact Apple.