The most obvious explaination is that it hasn’t been spidered recently, which is easy to fix, just link to it from your most crawled page. Other reasosn can be duplicate versions of the page, either your own or people who have copied it. If you haven’t made a change to the page recently, do so, when it gets recrawled it gets algorithmically reevaluated like any other page, so you could get back.