When you die in Minecraft occasionally you will be locked out of your game on the generating world screen as seen in the screenshot below. There is nothing you can do about this other than restart your game. It doesn't matter how you die; this has happened to me when I've been killed by a player, a mob, or from falling into lava. In the screenshot I'm using a health showing pack but even without any packs or game modifiers it still happens. Being on single player or multi player doesn't change anything, still happens. This is especially annoying when I'm hosting a world for my friends to play on and then I die and need to restart the entire game. I do not know how to replicate this bug as it seems to happen randomly when you die.