Boyfriend suddenly stopped calling/texting after things seemed to be going fine.?

I have been with my boyfriend for about 8 months. Since the day we met, we have talked every single day. Even if he was busy, he would always make the effort to talk at least once a day. Things seems to be going fine until one day he just stopped communicating. The first couple of times I asked him why he didn't call or text me back and all he said was "Sorry." It has been 4 days now and I have not heard from him at all. He has been updating his FB status regarding football games, so I know he is alive and well. Question: How long should I wait before I attempt to talk to him again and when I do, what should I say? Thank you for your help