My best friend is mad at me and is refusing to talk to me

I have two main friends, we all talk sh*t about each other on occasion, however we have mostly stopped. Recently we've been trying to be more honest with each other and try to resolve arguments instead of just holding a grudge. However, recently one of my friends has cut me completely out of her life. With nothing more than a vague notion of something I did having hurt her. I tried to talk it out, but she told me that I wouldn't be able to understand. I asked what I could do to help and heal our friendship, and she only told me to wait. I can't see her, and I fear I've completely burned a bridge down. She told me that I could try to message her all I wanted, but she wouldn't care. The thing is, I have no clue what I did.. Her parents don't like me, sure, but I really cannot think of anything I could have done. I don't know what to do..

You said 'vague notion'. What is it? And what does your other friend know or suspect? Is she still talking to and hanging out with this mutual friend?