I have a MacBook Air and my internal camera is not working.

Try rebooting the laptop.

Check that the camera hardware is detected by macOS by Option-clicking the Apple menu and selecting the first option. Click on the "Camera" item on the left pane of the System Profiler and look in the right pane to see if any hardware information is listed for the camera.

What app are you using with the camera? Make sure the app has permission to access the camera in the Security & Privacy System Preferences:

Control access to the camera on Mac - Apple Support

If your built-in camera isn’t working on your Mac - Apple Support

If the camera hardware information is not listed in the System Profiler and the app(s) have been given permission to access the camera, then you can try reinstalling macOS over top of itself, or the more drastic option of a clean install (erasing the drive & reinstalling macOS -- testing the camera before installing third party apps or restoring from backup). Otherwise the laptop likely has a hardware issue of some sort which will need to be repaired. You could use an external USB camera instead or temporarily.