In the Dragon Riders of Pern book series they abandoned their technology because they were forced by severe earthquakes and other environmental factors. At least at the end they were rediscovering the lost technology.
I wish someone would do a Pern TV series. Anne McCaffrey could spin a yarn, and her son isn’t doing too badly.
Volcanoes, illnesses that wiped out huge swathes of the population every so often and cost them knowledge, and sheer bad luck. Yes, I’m a Pern geek.