Maybe you should start a conversation. If he won't reply there is something in his mind that he cannot explain.
I'm having the same problem me and my crush used to talk all the time we have like almost all our classes together and in one class we even sat rite next to each other the teacher moved him because he talked to much so everyday when I came to class he would ask whats up but he never talked to me in the hallways then one day he said hey to me and my friend in the hall but in like a really quiet voice and at the same time I was saying hi to him a couple days later when I saw him in the halls again as we were passing each other I said hey and he just like walked past me and put his hand up behind him as if to say hi or something he didn't look at me or even talk to me he like shafted my life and lately he wont talk to me at all he never says hi but every now and then I think I see him looking at me maybe its my imagination but I just want to no y he wont talk to me what did I do I don't even care if he likes me or not but he wont even talk to me as a friend! All his friends talk to me and say hi to me in the halls and stuff y wont he?
He probably has a girlfriend or really not interested in you...get over it.
Maybe he's busy or doesn't feel like talking. Or maybe he just doesn't want to. Give him some time and space and whenever he feels like talking to you he will.