He's ignoring me!!! help!?

Okay, so there's this guy that I really like. And I think he used to like me, because he always complimented me and told me my hair was really nice and we used to be like almost best friends (that was in junior high), now we're in high school, almost a year already. Anyway at the beginning of the year we were still like best friends, but because we were in completely different cles in everything, we didn't see each other that much anymore. I sometimes see him in corridors and stuff, but he ignores me now, why? He used to always say hi to me but now it's like he doesn't even know me. Please tell me what I should do. thanks.