(((((((((( 50guy )))))))))))))))
Being together through thick and thin for 36 years is one heck of an accomplishment that I hope you can both be proud of. As in any relationship, there are many ups and downs and storms to weather. I would suggest that the best way to deal with this is constant communication between the both of you.
People change over their lifetimes....things that were once important sometimes don't seem as important as we get older....and vice versa. We look at life through different eyes as we age. We sometimes think of what we missed out on in our younger days and may try to recapture it "before it's too late". And sometimes there are things that may have bothered us through the years that we don't talk about and all of a sudden it weighs on our minds so much and becomes an issue, or we just seem to get to the end of our rope with it.
I've known a lot of folks who have been married/together for many many years that go through situations similar to this. I think of it in terms of having growing pains. If the communication and intimacy (not just sexual) can be accomplished, it's very possible that you both can weather this storm in time. I wish you both well!

sabby