I tried to open apps like Photo Booth and FaceTime on my MacBook and the build-in camera does not turn on. When I am checking the system preferences on the camera options it will turn on but when opening apps that require the camera access it will not work even thought on the setting it says it has permission to use. On the FaceTime app it says "FaceTime has not received any video from the connected camera." Restarting it has not helped either.
If that does not help, then let’s see if this issue happens in Safe Mode:
Shut down your Mac.
Press and hold the power button on your Mac. As you continue to hold the power button, your Mac turns on and loads startup options. When you see Options, release the power button.
Select your startup disk. It's named Macintosh HD, unless you changed its name.
Press and hold the Shift key, then click the “Continue in Safe Mode” button below your startup disk.
Log in to your Mac. You might be asked to log in again.
If the issue doesn't continue in safe mode:
Leave safe mode by restarting your Mac normally, then try to reproduce the issue.
If the issue doesn't return, it might have been resolved by the other things that safe mode did at startup, such as checking your disk and clearing caches.
