Black screen : écran noir

2 days ago, when I went for à zoom meeting, no more sound and no more video. The sound came back by testing it. But not the video : the screen is always black. Zoom tell me :"choose another camera", but I have not another, only one, and so, I don't know what to do.  Until now, for more than a year, all was always ok, but now, no. 

So thank you if somebody can help me.

I am french, I speak a little english and I hope I understand the english explanations.. Thank you for helping me.