Hi Christopher,
Has your wife been to her Gynecologist? She probably needs hormonal therapy so she can enjoy sex again . Have you been to a sex therapist also to help you both?
You need to talk these things out so you will understand where she's coming from and the reason why she shuns sex with you. Are you still virile ? Are you about the same age as her?
You two are the only ones who could decide where your marriage is going. Marriage without sex is miserable to either of the couples.
Take care and try to understand your wife. Don't consider cheating on her. This will surely ruin your marriage.