crash before startup

Put the summary of the bug you're having here

What I expected to happen was...:
i expected minecraft to start up as usual

What actually happened was...:
i clicked the online button on the minecraft launcher and it said minecraft has crashed

Steps to Reproduce:
1. double click the minecraft icon
2. click online when connected to the internet
3. the window automatically closes for a moment then opens again with a message at the top saying minecraft has crashed.