We are given only sparse information concerning "The Ruin," as it has come to be known in this excellent novel. It is clear that the precise nature of the destruction that mankind brought upon itself has been forgotten, but there are vague hints that we can pick up from the song that the Singer sings each year, charting the history of mankind, that we can use to suggest a possible scenario. Note the following terrifying details of The Ruin that are given in Chapter Three, that are so horrific that even though the whole community is supposed to listen to The Ruin Song every year, mothers will cover the ears of the very young at this point:
Verses told of smoky, poisonous fumes, of great fractures in the earth itself, of the way huge buildings toppled and were swept away by the seas.
We can either infer therefore that civilisation as we know it ended in a nuclear apocalypse or some kind of environmental oblivion, characterises by massive earthquakes and tsunamis that wreaked havoc with earth as we know it.