Minecraft started crashing after I started playing version 1.6.1

Put the summary of the bug you're having here

What I expected to happen was...:
Describe what you thought should happen here
I started the Minecraft application and thought i would be able to access my survival world
What actually happened was...:
Describe what happened here
The application crashed when i tried to open up my survival world

Steps to Reproduce:
1. Open up the application
2. Click on main survival world
3. Application crashes