HE isn't sexual?

My boyfriend and I have been together for over a year now, and lately no matter how hard I try, he doesn't want to have sex with me. No matter what, whenever I try to be intimate with him, he tells me that he doesn't want to. It's so incredibly hurtful to me, and tonight I finally asked him why he doesn't want to and all he said was "I don't know", which has left so much worry in the back of my head. Has anyone else experienced this? And if so, how did you handle it and work on bringing back the intimacy in your relationship?