this problem occur when Camera and Mic are blocked.

a. To unblocked

1. click the video icon that is placed at the right side.

click always allow to access Camera and Mic.

click Done to save changes.

click dismiss then refresh the webpage.

now you can try the mic and camera.

2. Another method is click the lock icon behind the search bar.

click Site settings.

click clear data.

select clear.

then after that just refresh the webpage and start the meeting.