Analysis: What's Up With the Ending?
At the very very end, Mrs. Murdo is walking alone. The message that Lina and Doon toss down from the upper world falls at her feet, and she begins to untie it (and, presumably, to read it).
Let's rewind a bit: Lina and Doon and Poppy escape Ember—woohoo!—but Lina's disappointed because they don't find the city she's been dreaming of, but rather this big beautiful place that we might call nature. And the kids realize that they've left Ember without telling anyone else about the details of the escape route, so that's another bummer. Things haven't gone quite as they (or we) expected or hoped.
But because they manage to get the message to Mrs. Murdo, there is in fact still hope. If someone like Lizzie or Looper or one of the mayor's guards had found the message, we would've thrown up our hands, because they wouldn't have done a darn thing with the message. But Mrs. Murdo, well, she's responsible and caring and trustworthy. If anyone's gonna read the message carefully and try to implement it, we'd trust her to do so (though Clary is a close second).
So the conclusion of The City of Ember is a tad open-ended, because we don't know exactly what will happen next. But we're hoping that Mrs. Murdo manages to lead everyone out of Ember to safety. Or at least to a better situation—a place that's not falling apart.