A good reason for this is probably she knows she can't get pregnant anymore. it "defeats the purpose" for some people who dont want to have sex just for fun. It's always possibly she just doesnt feel "sexy" anymore. Giving her self confidence might help.

my b/f is 51 & he says he says his sex drive is not the way it used to be, neither is mines.
i m on a lot of medication and it decreased my libido. so my question to you is, is your partner on a lot of medication?
does she have any health problems?
when was the last time that she was interested in sex?
my point is you to find out the reason why so you can try to rectify it.

I read this and first thought was-- maybe she's tired, overworked, has medical issues and meds that make her feel run down..Then I thought if she is not feeling the need anymore are you doing anything to find out why or are you doing anything to help her..How about giving her a massage and not asking for anything back..How about a romantic dinner..Could you clean the house for her buy her some flowers make or pick up a nice dinner so she doesn't have to cook..Why not court her..It wouldn't hurt to try..Why not write her a love letter and over a few days do MANY nice things for her..Get the twinkle going..