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 caus sex to become painful.
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.
I would like you to have a look at the Mayo Clinic's take on this here. It is very informative and in plain language. Share this with your wife and see what she thinks.
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.