I think it can be either fixable or not fixable. There are so many things that go into attraction to a person. Many marriages and relationships lose the "spark," but often find ways of reigniting it. Sometimes a person you're not physically attracted to at first, becomes attractive to you later, often after getting to know the person. Personality can often win out over the physical. Sometimes it can't. What you desire physically can change, so that's harder to change (back), but if you're in a committed relationship, I feel you should work on the relationship, to see if you can get back your desire for the person. In most cases, as several people have said here, it's because you're no longer happy in the relationship. Sometimes it can be fixed, sometimes it can't.