Why does my best friend doing this?

I've been friends with her for four, almost five years now. We are very close with each other & we tell each other everything. But one day she just texted me "i can't explain anything to you but I just need time alone and that's all I have to say." Then she blocked me on Facebook. Now she's always staring at me during school. And then one day she just decides to casually talk to me in texting & I later asked her 'why are you ignoring me and why did you block me?' and she had no response. Then someone else just told me a lot about her that she's done and now she has a boyfriend which who I warned her about. So what should I do about her? She's my best friend but it really doesn't seem like it anymore ..