Wattpad is a popular app for book lovers because of its ease of use and collection of novels, comics, etc. Unfortunately, Wattpad does not allow users to search for a book by its plot. The only way to do this is to use different sites such as Reddit and Quora and hope that a user can help you. Other than that, you can try searching for the book using similar tags or the author's name. Furthermore, you can try contacting the author to see whether the story has been removed.
If none of these work, you can always try a search engine such as Google.
A great tip is to archive all stories you have read. In this way, you can find the story easily if you want to read it again in the future.