Why doesn't he reply anymore??? 10 points!?

me and my boy mate used to be very close. we told eachother most things and trusted eachother and had fun over facebook inboxing eachother daily and i used to get my hugs. but then it all stopped.!? he's stopped inboxing me, replying to my inboxes, and apparently he called me a prick behind my back and said me and my mates act all hard when i litterally share the same friends with him!? i swear i haven't said or done anything to him, its just all of a sudden random thing !? he's changed because he hangs around with the wrong people nowdays!? i told him once ages ago and he told me he doesn't care and that everyone has to change so i never argued back, but this time, its gone too far? what shall i do to get him back? i even inboxed him saying why he's avoiding me and that i'm sorry for anything that i have done wrong though iam not so sure of what mistake i've made?!