Simply refresh your page, often re-establishing the connection between browsers solves these problems
Please see, if you opened the Virtual Classroom in Chrome or Firefox browser. If not, copy the link of the Virtual Classroom from the browser bar on the top of the page, open the Chrome or Firefox browser and paste the link to the browser bar, then hit Enter. This will make you rejoin the classroom. If this was the source of the problem, in your new browser the audio and video sharing should work.
It's worth to check what you are sharing with the other participants in the bottom left corner of the site.
If the problem is still present, copy the meeting link from the browser bar and close then open the browser again. Paste the link into the browser bar and rejoin the class by hitting Enter.