Minecraft Launches, but now crashes when I enter ANY of my worlds. (All previsouly work well)

Put the summary of the bug you're having here

What I expected to happen was...:
Enter into ANY of my previously saved worlds.

What actually happened was...:
When entering world, now always crashes immediately.

Steps to Reproduce:
1. Launch Minecraft Launcher
2. Select ANY user profile
3. Launch using single player.
4. Crashes everytime.