Well, the last time this happened in the West, it was the conquest of Hellenistic civilization by the Romans, if we believe Lucio Russo's account. This was a gradual process that took centuries, but two notable events in it are the clear-cutting of Plato's Academy by Sulla in 86 BCE (whose soldiers also killed Archimedes) and the lynching of Hypatia by Christians in 415 CE. Western science didn't recover from that until somewhere between the 18th century, when Harrison solved latitude and Lavoisier began cataloging the elements, and 01966, when the Roman Catholic Church ceased to promulgate the Index Librorum Prohibitorum.
Despite the Dark Ages in the West, science did continue elsewhere in the world, and even progress, but did not reach the general level of Hellenistic science and technology until it resurged in the West after the Renaissance, centuries after the Dark Ages ended. Many places had very stable societies—Ming China, Tokugawa Japan, Australian Aborigines—without it.
So, how can we go back? There's no guarantee we can. But we will go on.