Randomly Disappearing Worlds From The Single Player Selection Screen

Okay so I have been playing on my single player world for about a week or so now and yesterday when I went to go play on it the world had completely disappeared from the actual single player selection screen. The folder with all of the data is still there in the .minecraft folder but in the single player selection screen it has totally disappeared for me. I don't know how to fix it. I have tried everything but nothing is working. Please help. I spent hours just building structures, messing around with command and command blocks, and I have even decorated to make my area look more pretty. My world was time consuming so this really stinks. My world was also called God Mobs just in case you were wondering.