If prompted click allow, this is not sending video to us it only tests the browsers connectivity with the camera.
Webcam not showing on the screen?
Things to try:
Make sure the camera is plugged in
If the camera you are using is a USB camera makes it is plugged in to a known working USB port, if the camera usually has some sort of indicator that it is getting power this is a good sign.
If it is a USB Camera, can you test it on another computer to see if it works?
Try different browsers
If you are a user of different browsers on your computer, try a different on like Chrome or Firefox.
Is another application using the camera?
If there is another application for example Skype running in the background this can interfere with the camera showing up in the browser. Close other programs that use the camera and try again.
If all else fails, don't download driver detection software, these are known for malware and ransomeware.
Test is compatible with all modern browsers on Windows/macOS/Android/Linux