At the end of the book, Janie is convinced that she was kidnapped, and fears that her adopted parents -- who she loves very much -- are actually the kidnappers. She tries, and fails, to get through her fears on her own, but the loss of a letter to her possible biological family convinces her that it is time for adult intervention. Her adopted parents, seeing all the information she has gathered, realize that their daughter Hannah lied about giving birth to Janie after joining a cult. In fact, Hannah kidnapped Janie from her real parents, the Springs. Janie is overcome with the emotion and pleads with her adopted parents to leave it alone.
"Then I'm not going," said Janie. "I've had enough emotion. I just want this to end happily ever after.
"Not everything does, Janie," said her father. "Hannah didn't. I don't see how this can."
[...]"It has to!" cried Janie. "Tell the Springs, Lizzie. Tell them it has to end happily ever after."
(Cooney, The Face on the Milk Carton, Google Books)
Janie's adopted mother Miranda, however, knows that Janie's real mother has been living the last decade in terrible agony; losing a child is, for a parent, the worst tragedy possible. Despite Janie's request, Miranda calls the Springs and hands over the phone to Janie. In the last sentences of the novel, Janie speaks to her real mother for the first time in her memory.
The conclusion of the book functions in two ways. First, it is a cliffhanger; it leads directly into the next book in the series, and allows the reader to be expectant and eager to continue Janie's story. The cliffhanger structure is a classic method of making the reader want more. Second, the end acts as a catharsis for all the emotional drama that precedes it; as the event is taken out of Janie's hands, she is finally able to relax, knowing that her loving adopted parents are working to help her. Janie has spent most of the novel under intense stress, and now with the revelations of her birth and the contact with her parents, she can finally begin to heal.
"The Face on the Milk Carton" does leave the reader with a cliffhanger. It does not really tell you what happens. However there is a sequel to this book, "Whatever Happened to Janie?" This book has similar happenings in that Janie is back and forth on what family she really belongs to. The Spring children want her enough that they follow Janie to get her to come back to their home, where they believe she belongs as their sister.