Make sure that you are using the correct equipment and browser (Google Chrome, Microsoft Edge Chromium) and that your camera and/or microphone have permission to access the browser.
Try to run this test and check that the camera and microphone is allowed in browser: https://tools.vonage.com/video/precall
Click on the camera symbol next to the url:
Sometimes another software can use the camera and microphone which could lead to a black screen in the videocall, make sure to turn off any other software that could interfere with your camera/microphone and restart the browser before entering the call.
If the above are configured correctly, test with a new camera/microphone, while making sure that this new equipment is not used by another software when you connect to the call.