He clearly doesn't like to express his feelings or frustration...some people are that way (I was that way for a long time.) Men like to retreat to their caves sometimes.
She needs to say..."Look...I can see you're not happy. I do not understand why. If it is something I did, please let me know when you're ready so we can work on it together. I'm here if you need me. In the meantime, I will respect your space. Come to me when you're ready. I love you. " Then she needs to give him space. Back off. Do her thing. In time, when he's ready, he will come back.
It is possible he may be suffering from depression or something like that...in which he may need to see a doctor or licensed counselor. But that's out of her control. Sometimes the best thing to do is to respect someone's space and give them time.