If that is a built-in webcam on a laptop:
1
Look at the camera lens above your screen, is there a small privacy slider that needs to be opened to access the webcam?
2
If that was not the problem, look at the function keys (F1 - F12) on your keyboard, if one of them has a camera icon, press that key or Fn + that key to unlock the webcam.
3
If the webcam is still not working, click your Start Button, then just type msinfo and press Enter, please provide a screenshot of the resulting System Information window.