Time to heal

Been wondering about a friendship that I had. Well, it was sort of a friendship. Hadn't talked to this person in a while ... Been some months. I sent him a birthday wish via facebook email and also said goodbye. I told him that he didn't have to respond even though I wanted him to. Hoped he would. He didn't respond. I tried calling but that didn't go well at all. I didn't even get past telling him who I was before I was hung up on. No call back, as if it was by mistake. I didn't call back or email or text. I just left it and him alone. There was no explanation or bad words or anything. It's just a day later, and the pain is still fresh. But, I know I need to take the advice of a friend and just drop it. There is something nagging at me to know what went wrong or why it happened. I just have this feeling that I will not find out since he will not talk to me. Been told that only an a**hole would do something like that.