If you are using Teams on a Windows device, open the Camera app, is your webcam working correctly in that app?
1
If it is, open the Settings App, then go to Privacy and Security - Camera and ensure all apps are enabled in there to be able to access your webcam.
2
If your webcam is not working in the Camera app:
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.
If that was not the problem, look at the camera lens above your screen, is there a small privacy slider that needs to be opened to access the webcam?