Some applications such as Microsoft Teams may wish to access your webcam and microphone. In newer versions of macOS you need to give applications permission. To do this:
- Open System Preferences
- Click on the "Security and Privacy" in System Preferences
- Click on "Privacy" along the top, then "Microphone" and "Camera". Find the application you want in the list, and tick the box
- Close out of the program that needs camera or microphone access and re-open it.