I just had the same issue - I was playing a world in Minecraft Windows 10 Edition when my game froze and froze my whole computer. When I restarted the computer and tried to play it again, the world was missing from the list! I had spent months on this
world so I didn't want to lose all that progress.
So, after some trial and error, I got my world back. First off, open File Explorer and go to:
%localappdata%\Packages\Microsoft.MinecraftUWP_8wekyb3d8bbwe\LocalState\games\com.mojang\minecraftWorlds\
In this directory, you'll see a lot of folders with strange names like "jLUCAlmQAQA=", "0HQAALXzVwA=", etc. These are your Minecraft worlds. You can probably tell which folder is for your missing world by the modified date but if not, each folder has a
"levelname.txt" file that contains the world name. When you locate your missing world, there are files called "level.dat" and "level.dat_old". I renamed "level.dat" to "level.corrupted.dat" (just to keep it around in case this didn't work) and renamed "level.dat_old"
to "level.dat". Restarted Minecraft and voila! My world is back and playable.
My guess is that at the beginning of each session, the last game state, "level.dat" gets copied to "level.dat_old" in case of corruption so restoring the old "level.dat" file restores the game to the last session before corruption.
Neill, THIS IS SO HELPFUL! Something similar happened to me where my laptop restart mid game. Can you tell me exactly how to to restore the world back into the game? I have the folder with the world in it, but renaming didn't seem to work. After more tampering
the rest of my minecraft worlds, the ones that didn't corrupt, disappeared as well but those are insignificant. Any help would be appreciated!