The knowledge of concrete vanished for centuries after the fall of the Roman Empire too
Can we duplicate roman concrete even now? Last I heard, it was a matter of debate. But I also remember reading, that the addition of volcanic ash strengthens the concrete immensely, and increases its usefule life by some large factor - so perhaps it has been done.
The recipe of “Greek Fire” which kept the Ottomans out of Constantinople for centuries, is still not known.