When opening a standard chance cube, an ender crystal appears, explodes and minecraft shut down. Then the save files are no longer in the save games folder. IDK if it is a possible reward wich it an awfull reward, or it is some kind of error.
I was playing Project Ozone Mod Pack.
Regards

Ozone is a hardcore pack and that death was probably your last life. Upon losing all your lifes in hardocre the world is deleted. Sorry for the slow reply, I got caught up in other things.