The answer to this question can be found in Chapter Three of this excellent dystopian novel. As Kira is summoned before the Council of Guardians, we are told about the role of the Singer in singing the Ruin Song, which acts as a kind of oral history of mankind and how they reached this point in time. Note what the text tells us about this Song and why it is called the Ruin Song:
The story of the past was filled with warfare and disasters. Most especially it was frightening when it recalled the Ruin, the end of the civilisation of the ancestors. Verses told of smoky, poisonous fumes, of great fractures in the earth itself, of the way huge buildings toppled and wereswept away by the seas.
The reason why this song is called the Ruin Song is that it points towards one single cataclysmic event in world history, the Ruin, that was instrumental in bringing humanity to the present position that it occupies in the novel. Although the precise nature of this "Ruin" is never detailed, it is strongly suggested that some kind of environmental collapse or nuclear holocaust.