Why is he being so distant?

Hey ya'll. My bf had been very distant lately... He hasn't been responding for hours or at all to my texts or returning my calls. He stayed at his apartment for winter break in the college town where we both go to school and I went home. Since then he's been like this. He hadn't done well in his classes this term, but he's never really done this before. We've been dating for two years now. I miss my love.... what should do? Any thoughts as to what's going through his head?