The City of Ember Tome 1

Ember is the only light in a dark world. But now its lamps are beginning to flicker. The City of Ember was built as a last refuge for the human race. Two hundred years later, supplies are running low and terrifying blackouts are sweeping through the streets. It's only a matter of time before the lights go out and never come back on again. When Lina finds part of a secret message, she's sure it holds a clue that will save Ember.
She enlists her friend Doon, and together they explore long-forgotten parts of their dying city as they race to solve the mystery. If they succeed, they will have to convince everyone to follow them into danger. But if they fail ? The lights will burn out and the darkness will close in forever.