What do you do if your best friend decides she is not your friend anymore?

first of all your 'best' friend will not just go off with someone else. if they do then maybe you need to go back in time and examine your relationship with that person, ask yourself some questions like,was he/she a good friend? was he/she fair to me? was he/she truly there for me when i needed them? does he/she respect me? if the answer is no then i am sorry to break the news to you but that person was never your best friend. if the answers are otherwise then before jumping to conclusions you should go and ask that person to talk and ask them what is happening with the friendship with respect to you and him/her.