A few of my Minecraft worlds deleted themselves

When I got home from school today and turned my laptop off then on (because it does this weird thing where the start screen won't appear) while Minecraft was running but I was not in any worlds just on the main sceen. I Loaded up Minecraft again and my main world and like one other was gone. The world I loaded up (which I thought was my main world) had a giant hole underneath me. Everything was gone but I was not falling. It took a while to get to the edge of the now square chasm because the movement was messed up. Then when on solid ground I could not move. I spent hundreds of hours on my main Minecraft world. The logs in my Minecraft folder have evidence that my world was there hours before. Please help.