Check if the camera is connected properly to your computer if you are using an external camera. Close all programs that may be using your camera including Skype, instant messengers (G-talk, Yahoo, Facebook, Messenger, or Windows Live Messenger), and then press “Ctrl+shift+R” to refresh the page. 

Also, make sure you have allowed the camera and other required permission for the site by doing the below troubleshooting steps.

- Click on the secure option or the lock icon which is at the left of the address bar.

- There you can find a flash, camera, popups, microphone, cookie location, and notifications.

- Please keep it as "always allow on this site".

- Reload the page.