game does not load world from title screen. crashes

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 click on minecraft exe
3. it goes to title screen with single or multiplayer
4. i click on single player and it takes me to my saved worlds.
5. when i choose a world it starts to load then goes out of the game in a box telling me game has crashed.
3. ...