When I open up minecraft, it won't allow me to play online. Even on offline mode, I cannot play single player either

When I open up minecraft, it won't allow me to play online. Even on offline mode, I cannot play singleplayer mode either

What I expected to happen was...:
the world would start up, and I would be able to play singleplayer mode normally

What actually happened was...:
I went into a created world, and it loaded for a little bit, then crashed

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. Open up minecraft
3. Go into a new world in singleplayer