Archimedes (ca. 287-212BC) described what are now known as the 13
in a lost work, later mentioned by Pappus.
But it awaited Kepler (1619) for the 13 semiregular polyhedra to be
(Image from this link.)
So there is a sense in which a piece of mathematics was "lost" for 1800 years before it was "rediscovered."
Q. I am interested to learn of other instances of mathematical results or insights that were known to at least one person, were essentially correct, but were lost (or never known to any but that one person), and only rediscovered later.
1800 years is surely extreme, but 50 or even 20 years is a long time in the progress of modern mathematics.
Because I am interested in how loss/rediscovery might shed light on the inevitability of mathematical ideas, I would say that Ramanujan's Lost Notebook does not speak to the same issue, as the rediscovery required locating his lost "notebook" and interpreting it, as opposed to independent rediscovery of his formulas.