World Not Loading

I updated my Minecraft game on my MacBook the other day. I created a new world, and used some of the new world customization tools. I clicked 'Create New World' button, and the loading screen says 'Building Terrain' but it never loads. And when I try to quit Minecraft, it doesn't quit. When I try to shut down, log out, or restart my MacBook, I get a message saying that Minecraft cancelled the restart/shut down/log out and that I need to quit Minecraft before I do so. But I can't! Please help!