@Kye the printing press came around the 16th century (Gutenberg) and eyeglasses came around the 17th (Huygens and other Dutch inventors).
I've used that example because the Middle Ages (especially the early period) were indeed a period when engineering skills from the Greek and Roman period were lost. It took us more than a millennium before we figured out (with Brunelleschi) how to build a large dome like the one used in the Pantheon, and the same goes for the art of Roman concrete production.
@blacklight @Kye The Jikji was printed in 1377 and there may well have been books printed even earlier that haven’t survived so although Gutenberg developed the printing press in the West (in 1455), it came around earlier in Korea.