I cannot load my singleplayer worlds or play multiplayer

Put the summary of the bug you're having here

*What I expected to happen was the world would start, because everything loaded up fine.

*What actually happened was I clicked play selected world on the world of my choosing and the screen went to loading and building terrain. Then the game crashes every time!

Steps to Reproduce:
1. Begin Minecraft
2. Select Singleplayer
3. Select a World