Board Thread:The Flash discussions/@comment-4036559-20181010183851/@comment-37162533-20181012005109

Barry Allen aka the Flash seems to have once had during the Crisis of Infinite Earths had rather than completely disappear from the comics in sacrificing to attempt running fast enough to trysaving his own universe and multiverse and instead ran from a universe and multiverse within the DC comics Megaverse ran through the universal, multiversal, and megaversal barrier from the DC comics megaverse into the universe of Earth-TRN242 within the Marvel comics Megaverse. And as result of the cosmically overwhelming traumatic experience acquired Amnesia causing him to lose his true secret identity and name. He both called himself Buried Alien and eventually/developed gave himself a whole new alias "Fast Forward" he never reacquired his memories. So that is why he had never returned to his original home universe, muktiverse, and megaverse. In adjacent to a brand new replacement Barry Allen being created along with the creation of the new multiverse. So there you go DC comics fans there is the answer to your question of where did the Barry Allen of the Crisis of Infinite Earths disappeared to.