My Zoom backgrounds disappeared

Have you checked the folder where they are stored? If they're still in the folder (I keep all of mine in a folder called "Background Images"), then you just need to add them back to the application itself.

Are you logged in to Zoom? You must be logged in for your backgrounds to appear. And if you have more than one Zoom ID you may have different backgrounds for each.

You might want to search for Zoom Forums and ask their members about it. It doesn't sound like a Mac problem.