OR Cherry: What a terrible situation to find yourself in. It sounds like you are feeling rejected because you are being rejected. There's not enough information to identify why. One possibility is that he has developed impotence and is trying to hide it from you. A second possibility is that he has developed a compulsive problem with porn. A third is that he is suffering clinical depression. A fourth is that there are problems in the marriage that extend beyond the bedroom, at least in his perception. A fifth possibility is that there is someone else.
Ultimately, he is the only one who knows the reason for his behavior. Therefore, my only advice is just to pick a low-stress time, when you are composed and calm, and tell him you need to understand what's going on. In order for that to have a chance of working, you have to be totally calm and prepared to hear whatever he has to say. If he sees you're upset, chances are he'll say whatever he thinks will make you feel better. If you want the truth, you have to be accepting and willing to handle whatever comes up. This is one of those situations where you shouldn't ask if you don't really want to know the answer.
A marriage counselor might help the two of you talk though this, but it's a long shot. I have asked clients why they were acting the way they were and they told me directly, where they wouldn't tell their husband or wife. That works only to the extent that both parties are willing to make it work. If he blatantly refuses to talk with you about it, then your choices become making peace with his behavior or leaving the marriage. You can really help him only to the extent he is willing to be helped.
If you find he is willing to talk, and you are willing to gather more information, this is a good place to get feedback on how to respond. It's anonymous, and you can bet you and he aren't the only one's dealing with whatever issue it turns out to be. I don't think you'll be able to do anything to improve the situation until you know what the exact nature of the problem is, and you can get that only with his cooperation.