Archimedes (ca. 287-212BC) described what are now known as the 13
Archimedean solids
in a lost work, later mentioned by Pappus.
But it awaited Kepler (1619) for the 13 semiregular polyhedra to be
reconstructed.
(Image from tess-elation.co.uk/johannes-kepler.)
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.
Best Answer
Just today, I read in the July 2014 Bulletin of the American Math Society, in the Mathematical Perspectives piece by Gerald Alexanderson, that "Lorenzo Mascheroni ... in ... 1797, proved that any [straight-edge and compass] construction ... can be carried out by compass alone. And that is where the problem stood until 1928 when a student browsing in a rack of books in a Copenhagen bookshop found a small book by Georg Mohr, an obscure Danish mathematician. It was ... published in 1672. It contained a proof of what was then called Mascheroni's Theorem. The contents of this volume had remained totally unknown."