Hi, I'd like to help you with your problem.
I am sorry that you are going through this right now. When a partner changes in a marriage and we have no idea why, it can cause feelings of confusion, self blame, anger and mourning. What you described, a feeling like you have been robbed, is quite common.
Expectations in a marriage, especially a long term one like yours, are what causes us to feel secure in our relationship. When something abruptly changes, it makes the marriage feel insecure and unsteady.
Oftentimes, men in middle age have what is called a midlife crisis. This has become sort of a cultural term for men who suddenly and often without warning change and lose interest in their marriages, jobs and other life activates that till that point sustained them.
What causes a midlife crisis is unclear. However, there is evidence that hormonal changes affect men at this age as well, and not just women. Because their hormonal changes are not as dramatic, they are often ignored.
Symptoms of a hormonal change in men include irritability, loss of sex drive or a reduction in interest, depression and a number of physical symptoms similar to what women experience in menopause such as night sweats, hair loss and weight gain.
I highly recommend your husband see his doctor for an evaluation, if he is willing. That will at least rule out any physical cause to his changes in behavior.
I also feel it is very important for you both to see a counselor together. But if your husband will not go, go yourself instead. A therapist can help you deal with your feelings and help you find ways to cope with your marriage.
Have you tried talking with your husband about what is going on? How he responds can tell you a lot about what is going on. Is he willing to work on the marriage, does he see what is happening, and does he want to fix the problem are all good questions to ask to see what he is feeling.
Some books you can try are How to Survive Your Husband's Midlife Crisis: Strategies and Stories from the Midlife Wives Club by Gay Courter and Pat Gaudette and We Can Work It Out: How to Solve Conflicts, Save Your Marriage by C. Notarius and Howard Markman. These are available at Amazon.com or your local library might be able to get them for you.
Hang in there. With some effort, you will be able to figure this out and work through it.
I hope this helped you,