Hi there and welcome
Can you open finder and open photobooth
Can you then see yourself in the camera?
If not, what happens?
ok, can you do the following please
1. Shut down the computer.
2. Plug in the power adapter to a power source, connecting it to the Mac if its not already connected.
3. On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
4. Release all the keys and the power button at the same time.
5. Press the power button to turn on the computer.
Then try the camera again and tell me if it now functions please