Minecraft just crashes when trying to load up or create a world

Put the summary of the bug you're having here

What I expected to happen was...:
to load up my world

What actually happened was...:
It started to load, said loading and building terrain then crashed

Steps to Reproduce:
1. Click play offline
2. Click on world already saved/created or click create new world
3. then seems to be loading, states building terrain, then crashes and sends you back to windows