Are you sending messages via one single platform? Is there any remote chance he hasn't been getting them? Or is it showing he's read them? If it was me, I would send a message along the lines of "You obviously don't want to speak to me at the moment, and I don't know why. I'm not going to carry on messaging you, but would appreciate an explanation if and when you feel ready." And then just leave it.
But, if my boyfriend had ignored me completely for two days, I would assume we were no longer together in his eyes. You can be as busy as you like, but I refuse to believe that anyone in the world is too busy to send a text either apologising for being busy, or at the very least saying good morning or good night. It's fair enough if you have a casual, laid back relationship and don't speak for days on end - but the fact that you're trying to speak to him and he's ignoring you isn't fair, and it doesn't indicate to me that he cares about you very much.