Be aware you do not want to pad out the novel with extra words, descriptions, or pointless dialogue.
1: The first thing would be to write out something to show if you had written it as telling. (See my Showing Not Telling post.)
2: Not stopping the story where you did and continue to another ending is an option sometimes.
3: You could add a plot twist. A new obstacle prevents the Main Character from proceeding. You’ll have to be careful and maybe work it all out with the novel’s outline before starting. You might have to rewrite scenes on entering and exiting plot twist.
4: Add another sub-plot could be an option. It has to be relevant to the story. Do more going off the theme of the book or more about a character.
5: Add new conflict between the characters. Even if they are on the same side.
6: Find a beta reader, and if they don’t understand parts, you may want to add more to clarify.
7: Look at another book someone wrote in your genre and see if they describe their world more than you, more dialog, or more plot. Write an outline for their story and compare it to yours.
8: Check out each character you wrote. Are they one dimensional? As in, do they only have one thing about them. Is there another layer you can add to your main characters? You know tall, dark, and handsome has to have a flaw. Add something else.