Please help me with this situation?!!!! This is really important!?

Ok so I have to volunteer with this one girl at the library, and last year we were best friends. We would email each other every day, and always talk, and then this summer then she completely turned one me. She changed, and she is ignoring me, and when I talked to her about it, she said that I was a liar and a poser and a loser, and that is not me. I am not a liar and a poser and a loser, shes just making this stuff up, and telling my friends this so that they won't talk to me. If you were to ask any of my friends this, then they would never say that about me. Please help me on how to deal with this. (And I also want to talk to her by Saturday, so can someone please answer before then?)