I’m afraid this is a complicated issue. A dozen different contributing factors are possible, and the challenge you and your wife face may be a combination of them. You’re not alone, which is, I recognize, both bad news and good news.
I’ve addressed low libido and many of the contributing factors in blog posts before. Here’s the introduction to the topic on our website; here’s a blog post with additional explanation. Here is a list of answers to specific questions readers have submitted, including certain health conditions or medications which can inhibit libido as a side effect.
You didn’t describe your wife’s overall health; I hope she’s had a recent evaluation from her health care provider and has mentioned her loss of interest in intimacy. He or she can help to determine the cause, which will then point to the best options for treatment. You may also find a couples therapist or a sex therapist to be helpful, since emotional as well as physical factors come into play in intimacy.
I do wish you the best with finding the cause and some ways to address it! Physical intimacy adds meaningful dimensions to our relationships.