One of the most famous examples of "lost mathematics" must surely be the whole subject of Galois theory and, to some extent, group theory as developed by Galois. Galois attempted to publish his work several times, but it was overlooked by the likes of Cauchy and Fourier (in his defence, Fourier died). Luckily nine years after his death Galois's papers found their way to Liouville and they were published.

Under a strict interpretation of the question, Galois theory is perhaps not an example of "lost mathematics" because it not recreated by some one else. On the other hand, it definitely was "lost" and it was then later "rediscovered" by Liouville.