Hi, I'm really sorry to hear this cos a similar thing happened to me with my best friend of 7 years, and she just suddenly started ignoring me and being really nasty. It hurt so much when it happened first and i tried to sort it out by ringing her to discuss it and rebuilding our friendship.
a few months on, I realise that I don't want to be friends with someone who would treat me so badly for no reason, especially when I would have done anything for her.
The thing that really helped me is that my other friends, who i wouldn't have been as close to, saw what was going on and really supported me and made me feel better and I feel like I've created stronger friendships through losing my best friend.