This is caused by one of these issues: You have denied permission in your browser to access your webcam. You haven't selected the correct microphone and/or camera. Another browser, tab, or application is using your microphone and/or camera. Does your cam work elsewhere? Let's check at: https://www.onlinemictest.com/webcam-test/ If they don't, you know it is a hardware issue that you will need to address.
More details: If you are using an older operating system such as Windows Vista or XP, these no longer have support from Microsoft or browser teams like Chrome and Firefox. Update your operating system to use chat.