I have a question... is your wife not interested in sex (as in no "desire") for sex, or can she no longer *enjoy* sex. So, if she gets started, does she get aroused and have orgasms? Or does she not have the same feelings anymore?
And has she discussed these issues with her doctor, or (better yet) a sex therapist? There are a lot of hormonal issues that can come up (anytime in life) with women that affect their desire, their sensitivity, their bloodflow, etc. And there are a lot of knowledgeable people out there that could help figure out what's up and see if there might be some recourse.
If it's low desire, well-- that happens to most of us at one point or another. Desire actually runs in two directions-- desire leading to sexual contact and sexual contact leading to desire. Some people are more one way than the other, and there's nothing wrong with either. But a lot of women think if they don't feel desire all by itself (getting horny) then there's something wrong and that means they don't want sex.
Mind you, if she's decided she doesn't want to work on this and is happy being without sex, that's a different story, and her choice.
I just hate to see people resolve to situations that are fixable due to lack of the correct information and resources.
In any case, if she is interested in finding out more she can find an AASECT (American Association of Sex Educators, Counselors and Therapists) certified sex therapist by going to the website.