Super flat world of 255 blocks of dirt is stuck on "generating world".

While playing in Minecraft, I love to use super flat worlds in creative. I created a new super flat world that had 255 blocks of dirt. When I go to create the world it just is stuck on "Generating World" I than have to do ctrl+alt+delete to get to task manager to end the game. This has never happened to me before.Please help. I have even tried to do this on previous versions.