I have had two situations where I have had a similar experience. One involved my best friend of 15 years, the other involved a childhood chum/classmate/casual friend of 25 years. Both cases also involved life changing situations that these people were involved in.
To put it mildly, one moment the friendship was going along fine, the next moment they had nothing to do with me. The contact was usually ended with an abrupt e-mail.
In both of these situations, my obsessiveness/interests in their situations collided with human dynamics and became issue not necessary with my friends mentioned, but those close to them including family, other friends and others involved in their lives.
Maybe your friend is going through a life experience that she does not want your presence in, or someone close to her has had qualms with something you said/did and her way of dealing is divorcing you from the situation.
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I know it hurts, but there is also hope. After about a year or so of cooling off, my best friend and I started speaking to one another again. The friendship has come back as strong as it was before. My casual friend and I are still having problems, even though he doesn't return e-mails, telephone calls, Facebook related correspondence (he also unfriended me), he has been cordial the couple of times that I have run into him out and about. I have not yet had the opportunity to discuss the situation with him, but I have heard through others that are close to him kind of what happened.
Just hang in there, and I hope all works out.