Sometimes there are medical reasons why a woman loses interest in sex. If you are fairly sure she is not seeing someone else, then maybe there is something going on like a loss or hormones. For example, several years ago I had pain around my vagina every time my husband (boyfriend then) tried to have sex. This wrought havoc with our sex life for around a year and a half. My gynecologist didn't want to hear it. I had thought the days of doctors saying "It's all in your head" were over, but apparently not. I switched doctors on the recommendation of a friend, got some estrogen cream and now the problem is under control.
You haven't said that your wife is experiencing pain, but my point is that there are physical reasons (often) for losing interest in sex. One thing that helped me a lot was reading information by Drs. Jennifer and Laura Berman. They are sisters; one a urologist and one a psychiatrist. They specialize in helping women to find out what is causing sexual problems, and treating them. I suggest you look at
http://www.bermansexualhealth.com and
http://www.drlauraberman.com.
At the time I was having problems, they had written a book called "For Women Only" that addressed a variety of common problems, and encouraged women to seek help from their doctors. It gave me the nerve I needed to keep looking until I found the right solution. In spite of the title, you might want to get yourself a copy and read it.
Also, this will sound very "Dr. Phil", but a relevant question is "Does she
want to want to have sex?" That is, we know she doesn't want to have sex, but does she wish her desire would return? This could be important. I hope for you both that the answer is yes. Good luck.