Q: missing camera

See "Verify your Mac meets the FaceTime system requirements" here:

FaceTime for Mac: Troubleshooting FaceTime - Apple Support

Use Finder to go to your Applications folder.

Open the Photo Booth application.

Does the Mac's camera respond to this application?

If so, the camera is working and you have some other issue.

If not, the camera is not working.  Take your Mac to an Apple Store or Authorized Apple Service Center for assistance.

One other thing:

Before going to the Apple Store, if you do have an issue with the camera, check the "Built-in iSight cameras" section of this article.  The article is no longer current, but it does contain some good suggestions.  iSight is an obsolete term for the FaceTime camera.

How to Troubleshoot iSight - Apple Support