Hi! This error occurred in a day when and I wanted to play my Survival world, but the game disappeared ... I despaired and began to discover what had happened. Then I went into .minecraft, then saved games and my world appeared, but I couldn't start it. Restart the initiator and my computer and it still does not appear, I would appreciate a solution, since I do not want to lose items, progress and construction that cost me a lot.
 
Thanks.

PD: THIS IS TRANSLATED FROM SPANISH TO ENGLISH WITH GOOGLE TRANSLATOR