I was working on one of my minecraft worlds, then I did save & quit, then I closed the tab. I went to work on my online homework, then I went into minecraft again. I clicked 'my worlds', then I saw a message saying something like "An error happened while loading resources". So I closed minecraft again and went in again, then I clicked 'my worlds', and I saw all 42 worlds gone. 

The weird thing is, that my resource packs and my characters are still there. My settings, avatar, minecraft friends, etc., are the same but it's the worlds that is gone.