FaceTime HD camera stops transmitting live video in Catalina -- how to fix?

At some point in Catalina's evolution (10.15.5+ ?) situations started recurring where there’s no video with the Thunderbolt Display attached and the MacBook Pro in closed clamshell mode when attempting a videoconference via FaceTime or other videoconferencing app (Skype, Zoom, etc). What is strange is that the Display FaceTime HD camera is selected and the green LED is illuminated. I also tried disconnecting the Thunderbolt Display to see whether the MacBook Pro’s built-in FaceTime would transmit live video — and it did. But then there was no audio! I was able to reconnect the Thunderbolt Display to regain audio, but of course there was no live video.

The workaround for this problem is to restart the computer.

Has anyone else seen this problem, and is there a fix besides restarting?

I know this is an old question, but just in case it helps someone else who lands on this page: it seems this is related to a bug with MacBook Pros after they wake up from sleep. You can deal with it by simply restarting the camera plugin. Close the videoconference app, open Terminal and type:

sudo killall VDCAssistant

Type in your password at the prompt and that's it. If the issue was related to the wake-up-from-sleep bug, then the Thunderbolt Display camera should be working once you start the videoconference app again.