My best friend fell out with me about ten years ago. I had no idea why, she just stopped contacting me, didn't return my calls or text me back. After a few weeks I thought stuff you and stopped bothering. It hurt like hell, I didn't understand why.
After a long while I had a text from her telling me there was a dress I would like at a shop in town that she had just seen! I replied, asked she was and she replied and that was as far as it went.
I missed her like mad, but was stubborn and thought it was down to her to get back in touch again. After nearly two years she did, I had a text that said how sorry she was and missed me etc. It made me cry. She rang the next day, we both cried on the phone and met up a few days later.
We were quickly back to being best friends again, but I still don't know what happened. She couldn't really explain why, we've skirted around it a couple of times when there's been too much wine involved, but I still don't really know.
Sometimes people do something that they regret, a fallout of some degree happens and then time passes and it becomes too much of a big deal to get back in touch.
If the same scenario happened to me again I would react differently and I would go and see her and wouldn't have let that time pass. If it still hurts you and you miss her, contact her and tell her she may well be feeling the same.