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.