Several possible causes.

1) If the camera is built into a laptop, make sure there is not a toggle switch to manually disable the camera.

2) If it is an external device, ensure that the camera is turned on and that necessary software/drivers were installed.

3) Check OS settings to ensure it is enabled:

• Windows: Settings > Privacy > Camera
• macOS: Apple > System Preferences > Security & Privacy > Privacy > Camera.

4) Ensure there is not some sort of software that may block the use of this type of hardware (for example, anti-virus).

5) Check to ensure drivers are up-to-date for the device.

Outcome

The webcam is now working.