What went wrong in The City of Ember?

What precisely went wrong in the fictional city of Ember, depicted in the novel or film of the same name? Was it a failure of the builders' initial planning, or was it a series of unforeseen events that led to the city's descent into darkness and despair? Were there any signs of impending doom that the inhabitants ignored, or was the situation completely unforeseeable? And what can we learn from Ember's struggles and eventual salvation that might apply to our own societies and economies?