World load stops at 87%

I started Minecraft, and before I went into the world select screen I backed up all my save files. I created a new normal type world, and the loading froze at: 'Preparing spawn area: 87%'. Nothing happened and I had to force quit the game. If I pressed the X (close window) button nothing happened, and it never said that the game stopped responding. I loaded the world later and it worked as normal.