It crashes and won't let me back online.

Put the summary of the bug you're having here
My game keeps crashing and it won't let me on minecraft! Please Help
What I expected to happen was...:
Describe what you thought should happen here
When I pressed play it would send me to the launcher again and it says it crashed over and over again.
What actually happened was...:
Describe what happened here
It took me to the launcher and said there was a bug. I tried again but it didn't work. I even restarted my computer and it didn't work!
Steps to Reproduce:
Put a step by step guide on how to trigger the bug here
1. I started the launcher.
2. I pressed play
3. It took me back to the launcher.