The civilization ended in a catastrophic war that included huge explosions and a "poisonous mist". It's important to understand the this story was written eight years before the atomic bomb was first exploded, so the author was somewhat prophetic in his explanation of society's end. However, John's explanation for the god's destruction was that they "ate knowledge too fast". In other words, they didn't know how to control the knowledge they had gained and it ending up destroying them. John thinks that his people can avoid repeating the mistakes of the old civilization. The author is thus commenting on the cycle of history where one civilization rises and then falls and another civilization takes its place. We don't know if John's people can avoid repeating the mistakes of the "gods", but given human nature, and John's excited comments about sharing his knowledge with his people, one would doubt that they could avoid the same mistakes.
Civilization ended by a huge war that contained explosives and other poisonout stuff.