Now for a bit about Endings that don't work and what happens. An ending that isn't satisfactory will leave the reader with a sour feeling and may lose this reader for future attempts. No writer wants to become a "one hit wonder" but there are many who do that. There are more reasons than a book with a bad ending. Some people have but one story to tell. Some people tell the same story again and again until the readers get tired of that story. Those aren't problems being looked at today. This is about bad endings.
There is the ending where the main character who has played a full role throughout the book only to have the writer put this character on the sidelines for the big moment. This will leave the reader with a number of questions and that sour feeling. "Why did I bother to read this story if the character I've identified with through the entire story has no role in the ending?" If this has happened to you while writing the story think of a way to get that main character into the ending of the story. If the hero or heroine is going to be rescued by someone else, make sure the hero or heroine goes down fighting and has a part in the defeat of the enemy, even if it's a small part. Small but significent is the rule. Otherwise find a way to have your main character be the center of the ending.