When I launch minecraft, it crashes instantly.

I'm trying to launch minecraft but whenever I try, it always crashes.

What I expected to happen was...:
I thought minecraft would log me in and run normally, like last time.

What actually happened was...:
When I click the "Play" button, it looks like it will launch, but the launcher will pop up again saying minecraft has crashed.
Steps to Reproduce:
1. launch minecraft
2. click play
3. watch as it crashes.