It's started 12/14/2013 and I don't know how it was caused or how to fix it

Put the summary of the bug you're having here

What I expected to happen was...:
Describe what you thought should happen here

What actually happened was...:
Describe what happened here

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...I opened Minecraft
3. ...I clicked play
4. ...It crashed