I'm sorry to hear that. It's very hard when friendships end, and even hard when no one knows why. Some people do just grow apart, but from what you said, it doesn't sound like that in this case. Would you be able to ask her to meet you for dinner or coffee because you miss her? and try not talk about the past issues.

Sometimes people try to distant themselves from people they are closest to for reasons we will never understand. If she is suffering from depression, she may not be comfortable with you knowning or in her eyes 'calling her on it.'