The first and quickest thing to try is to reload the page in your browser, to see if that sorts our the issue.
Close all tabs in your browser and then close the browser app, before then opening the browser again. You can use the "Save and Finish Later" tool to get you back into the check quickly.
There may be apps running on your device which are using the webcam in the background, which may be interfering with the process.
If using Internet Explorer, then try another browser such as Chrome or Firefox.
You can also try restarting your computer entirely.