My phone suddenly shows an SD-card notification

My Sony Xperia J phone was in my pocket and I wanted to read an ebook, so started my reader application. It showed a black screen and when I checked the shelf, all my books were gone. I noticed a new notification icon that looked like a folded SD card. I checked its message that said: **SD card is empty or its file system is not supported**. What? Half an hour ago I was using it. I tapped the message and a question appeared: *Do you want to format your SD-card? It'll erase all data from it.* What?? All my ebooks and media are gone just like that? I restarted the phone, still no SD-card. When I got home, I removed the card and reinserted and fortunately now it works. How does that so-called OS dare to lose my data? And the "recovery" solution is to _format the card_? What could cause this?