How to win back friend?

For some reason my best mate has stopped talking to me and has stopped replying to texts and has removed me as a friend on Facebook! I don't no wat I did wrong! It kinda started when he started talking to a person who I hate and hates me back and it got worse from then on! I'm not too popular and I onlyreally have this friendship I tried inviting him over for sleepover but he hasn't replied to my invitation! I really wAnt us to be friends again ! Wat should I do please help me?!?