Hi Girls and Guys
Tags: savegame gone corrupted save file world in save but not in game
Problem : your save game does not not apprear / is not listed anymore in your game.
Why? it happens when you are playing and your computer get a blue screen, crash , or reboot the level.dat in your save folder get corrupted
How to fix it:
rename your not working "level.dat" into "level.dat.old" (just in case for you)
take a "level.dat" from an another save folder (a new one for example like ") and copy / paste it into your save folder world.
(note: the save game files are stored in the folder "saves" of your ".minecraft" folder , in which you can access by pressing the window button on your keyboard + R and type "%appdata%" then press enter, and go into the ".minecraft" folder)