Minecraft worlds are gone

I was playing Minecraft, then my laptop went off. (I don't know why) When I turned it back on, I wanted to keep playing Minecraft, but my worlds were gone. It was just the worlds I made at the very beginning that were still there. I've been working on one of these worlds for a long time and so it would be nice if we could fix it, as this is my 3rd time experiencing this. Can you get the worlds back