minecraft will not start up it crashes

Put the summary of the bug you're having here
minecraft crashes when starting up
What I expected to happen was...:
Describe what you thought should happen here
it should have loaded up i would like it sorted please
What actually happened was...:
Describe what happened here
i started up minecraft and it loads the update log and then i click play and i crashes
Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...
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