I've had this happen to me. One of my friends that I had known from second grade did this. It started out as just they'd take a day or two to reply to my texts, and then it'd be a week or two, and then they'd only talk to me once a month when we passed by in the halls or something and then we just stopped talking altogether. Really sucks when it happens. They never told me if I had done anything wrong or if they just didn't like me. We just grew apart and with that a bunch of my other childhood friends grew apart from me as well.