Can you please help me?

I was in love with a guy but after a while he left me without telling me why. I was deeply saddened by this and always blamed myself for it. Now I really want to talk to him and ask him why he did this to me but I don't know how I should approach him. He is completely ignoring me but I just want some closure to this situation I feel like I'm left in mid air. He was really kind to me but after a while everything just changed. I did tell him once that I feel very lonley and promised that he's gonna be with me but he never lived up to that. please tell me what to do.. :(