There could be so many reasons why he is behaving this way—financial stress, an affair, psychological problems. The fact that he refuses to talk about it is worrisome. He may also be ashamed of something he did (other than an affair).
When people refuse to talk, it’s often because they think the person who wants them to talk will come down hard on them for whatever it is that they don’t want to talk about. It can be helpful if you express to them that you can accept hearing about anything they do, and you will listen without criticizing.
Then you have to listen. When you hear what he says, I hope he will also listen to your feelings about this. Always use “I” statements instead of blaming him for what you feel. If he has had an affair, you should say, “that really hurts me. I’m afraid you don’t love me any more and that you’ll leave me,” etc.
You don’t say, “You’re hurting me. You’re making me feel so much pain.” That is a blaming approach, and even if it’s true, it doesn’t help foster openness.
Let him know that you are worried and that you care about him. Let him know that you think this is a relationship that goes both ways, and you want to know what’s going on so you can figure out what to do. You want him to be honest with you. That means you have to hear him out, and not stop him with all your feelings and blame before he finishes.
If you can’t do this, then counseling can help you. If he won’t go to counseling, then, despite your love for him, you may have to face the fact that something has come between you and you may not be able to remove whatever it is. You will have to decide if you want to continue in a relationship with someone who is not honest with you.