your mirror button came off and took a small chunk of glass with it. This compromised the inside layer of the windshield, (windshields are 2 layers of glass with plastic in between; it's why they don't fall apart into a million pieces and stay together somewhat) and it started cracking from there. most likely, it's not repairable since it's already that big. If you do get it replaced, you can have the technician take the button off the new shield and make a bracket for the mirror coming off the roof...or run without it entirely. I was a glass tech for 3 years, sometimes you have to swap mirror buttons for different years and different car options; its not hard for them.