My ad was Sticky. Why did it disappear?

Your ad did not disappear. As described at How do I get my ad at the top of the list? (Sticky ads), Sticky ads are selected on a variety of criteria. They can be “Sticky” for a week or two, sometimes more. When removed from the sticky list, the ad remains searchable in the category in which you placed it, until you delete it or until it is excessed by the code system.