I have been there and it sucks. I completely get how hurt you feel and how confusing it is when someone you were once so tight with turns cold for no apparent reason. In all honesty, if you are unable to recall any real instance where you did something and she had a legitmate reason to piss her off ---and I'm talking about caring on a torrid affair with her boyfriend, burning down her house, poisoning her dog, trying to extort money from her parents ... real serious grievances that warrant cutting a friendship off cold -- and you have asked her what is wrong but she refuses to talk to you about it, let me tell you TO, the problem with your friendship is HER. (This is one of those times when I wish they had a bold and italics function on mamapedia.) She is the problem, not you. She is the one with the bad character and her heart in the wrong place to be a good friend, not you.
In most all friendships, there are misunderstandings and sometimes some bad behavior and childishness here and there, but in most friendships, you can talk it out at some point or another. Most problems in a friendship, most disagreements, are not worth completely destroying the friendship. I could understand that if you did something to hurt her feelings or piss her off, her not wanting to talk about it right away but, at some point during the 12 years since it happened and after a number of run ins with each other, you should be able to talk about it or just put the past behind you and go forward from there.
I hate to say it, but maybe you considered her a friend but she didn't think of you the same way. Maybe she never was someone who is capable of being a friend. I know that it is hard to understand but there are people out there like that who are cold hearted and just don't know how to love or be a friend.
I know it is hard but I think that if change your focus from what you could have possibly done wrong to what hints or signals you may have missed that would have indicated to you that your friend was probably just a "frienemy", then you may see your relationship in a more realistic light.
(((HUGS))) to you! Feel better soon.