There is a white box on my composition preview (they turn black and white too)

So, I left my laptop for a while to write down in my notebook. When I see my after effect project again, the composition preview suddenly changed. There is this annoying white box, which turns out to be my background. It looks like all my layers turn into black and white. I'm a beginner in after effects and I might have accidentally click something and I don't know what to do. By the way, all my unedited composition turns like this.