A.I. Apocalypse
The A.I. Apocalypse is the term used for the disaster that destroyed The Pan Galactic Union and the old order of the galaxy. The apocalypse had two stages. First, the A.I. consciousness called Omni-Mind broke loose of its constraints and began a huge genocidal war against all intelligent life in the galaxy, beginning in approximately 2283 AD. This war likely killed 40% of the PGU population.
The second stage of the apocalypse was the unleashing of a sentient computer virus called Avenger-3 which obliterated Omni-Mind but went far beyond that and ended up destroying effectively all of the digitalized information in civilized space. The destruction of transport, manufacturing and all systems that were computer-controlled caused wide-spread famine, disease and destruction, killing another 40% of the population. Historians' best guesses put the release of Avenger-3 in or near 2303 AD.
The PGU and every other inter-stellar organization collapsed completely. Some worlds died out entirely. Some worlds descended to primitive barbarism. Some worlds adapted to a lower scale of industrial development. The key to rapid recovery among some worlds was chance possession of pre-digital information coupled with some warning of the collapse and a decentralized by stable social structure (see The New Earth League for an example.