Similar has happened to me, a woman who was previously my best friend (we no longer live in the same country but spoke every day, including for a few hours every week on skype), then for no reason she just stopped replying to me or if she did it was just one word answers, never asking me anything. I have absolutely 0 idea what happened, we didn't argue or anything.
Sorry no advice I just wanted to vent, it is so odd and is actually worse than a romantic breakup imo, it is so strange to lose someone you have shared so much with