"Has anyone else had the problem where your wife just seems to lose all interest in sex? Is there a physical problem, or is it mental?"
Dear [/B]underpressure,[B]
Your questions (above) sum it up. I doubt if anyone here can answer the questions. As to whether your wife has a physical problem (possible but not likely), only a thorough examination by a physician can produce an answer.
Having determined that your wife is healthy, the two of you should go to couples therapy. Often, just a few sessions will work wonders. Phone a large hospital in your area and ask to speak to a social worker who will then be able to refer you to a therapist who can help.