Minecraft not working

Put the summary of the bug you're having here

What I expected to happen was...:
Describe what you thought should happen here
I expected the game to work. It wont open with error messages showing.

What actually happened was...:
Describe what happened here
Error messages stop the game from starting. It has been used numerous times before but now wont work.
Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...Click on short cut or game but it wont open. Error message appears.
3. ...