All of my slides have thumbnails, but they aren't visible when I view them individually. Where did they go?

I'm working on a course and all of a sudden the slide I was working on disappeared. All that's there now is the gray background that normally surrounds the slide. I checked the other slides in my course, and they're all the same. I can't edit my slides because I literally can't see them. Does anyone have any idea why this happened? It happened to my coworker yesterday, but it was only one slide. This is my entire course. What's going on?

 

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