This is in fact a very common situation, and requires some VERY gentle handling.
There are many, many reasons why women lose interest in sex – some physical and some psychological – and for the most part, they can all be treated.
Hormonal changes, especially around menopause, can cause sex to become painful, and menopause too can produce some psychological symptoms that result in a lowered sex drive for women - often issues around self-image, and what is happening to her womanhood.
While I understand that she may be a bit reluctant to talk to her Doctor about this, let me assure you that he has seen it all and heard it all before – many times! He is only there to help, and should be her first port of call, to make sure that there is no physical reason for this lack of interest.
On balance of probability, it is most likely that the cause is more psychological and than physical, but it is important to rule out the physical first. Stress issues, changes in her lifestyle or relationship or hormonal problems could all play a part.
When that has been done, you can explore the psychological issues that might be at work, and first, I'd like you to have a look at this website:
She needs to understand quite clearly how this is making you feel - unloved, unwanted, rejected I suspect. This is something you both must talk about, but calmly and without blaming or shaming.
However, bear in mind that this is a very difficult time for her, emotionally, physically and hormonally – so above all else, be gentle and don’t try to force things. Be kind and encouraging, and suggest that it would be good for your relationship if she was to get help from her Doc. That’s the first step on the way to solving an all too common problem.