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 if you have your video and audio on, you can see it in the bottom left corner, next to your camera image.
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.
If the connection is weak or you see the "Internet problem" bar in the room try to turn off your video and use audio only.