It happened to me many times, also in earlier versions (I lost many worlds).
 every time after death when I choose to exit the game (to the main menu) instead of respawn I lost the world for ever and I can't enter the world anymore and stuck in generating world (Building terrain). 

so it is like that:
Playing (survival mode not sure about the other modes)> Death > ---two options "Respawn or go to main menu" >choose main menu> try to enter the world again> endless building terrain. 

It is a bug, I have encountered this many times, I don't use any  extra resource Packs or anything.. I am starting to hate the game because of that. please fix it!

Please I want to be able to enter my world again.