You didn't mention your husband's age. Things that come to mind for me (as an older man) include: depression & / or anxiety... (possibly related to work?) prescription medications, erectile dysfunction (or fear of it), loss of libido due to approaching (or entering) middle age (mid-life crisis), low testosterone, guilt over something you, perhaps, don't know about (an office affair... perhaps over but still causing your husband to feel guilty but not being willing to talk about it with you?)

The one thing I think I know is, if your husband won't talk about it and won't seek help for whatever is going on, there's perhaps little or nothing you can do to remedy the situation. My best wishes to you both though.