Zero Hour was a holocaust-level crisis in which Parallax attempted to wipe out existence, in an effort to remake the universe to his liking- a universe where, as he said, "Everybody wins!" He accomplished this by manipulating the connection forged by Krona between the entropy at the End of Time and the creation at the Dawn of Time, causing time to be wiped out from both ends- past and future. As this happened, temporal anomalies (aftereffects of the Crisis on Infinite Earths) began to manifest: among other things, Steel met his younger self, legions of alternate Hawkmen began to appear, Superman met the Jor-El and Lara of an alternate Krypton and Batman found himself in a world where his parents lived, but he died.
Initially, the Linear Men and the heroes of Earth believed Extant was responsible for the events, until Parallax revealed himself just as the present ceased to exist. A handful of heroes survived outside time, who were able to defeat Parallax and create a new big bang using Damage. The resulting history was largely the same as the pre-Zero Hour "Earth-Sigma," with most differences being very subtle. The primary difference was the total erasure of the Legion of Super-Heroes' history, restarting their adventures anew.