Without context it could be any number of reasons. Longevity of relationship may have led to a reduction in desire, change in connection or establishment of routines that no longer stimulate. Age may be a factor concerned with sexual desire, ability, and hormonal changes. Contraceptive use could be an issue, with some hormone treatments leading to changes in scent and reduction in desire at an unconscious level. The list could go on and on.
Ask yourself what the nature of the connection is, perhaps it no longer requires or needs or inspires sexual energy. Nothing wrong if it doesn't. Then ask whether you need it in your life. Nothing wrong if you don't. If either of those answers wants to be switched to a positive, then have a frank discussion and see about coming to some ideas, perhaps with help of a professional.