IMO, the first step is the check to see if the MPN you put in the item specifics is correct. I would then check previous solds of the same item to make sure the category is correct.
If those all check out, then open the listing to revise and move a picture or do some other small change and submit the changes. Recheck after a few minutes to see if the listing is now visible. For some reason occasionally I've found there will be an indexing error where the listing will be active in the database but disconnected / orphaned from search. Revising and resubmitting almost always fixes the issue.
