In the 1920's Ernst Steinitz discovered a remarkable theorem which today is known as Steinitz's Theorem - A graph G is isomorphic to the edge-vertex graph of a 3-dimensional convex polyhedron if and only if the graph is planar and 3-connected.

Only in the 1960's did the importance of what Steinitz had accomplished become clear when Branko Grünbaum and Theodore Motzkin recast/rediscovered what Steinitz had done in modern graph theory theory language that the importance of this remarkable result came to be exploited.

I would also like to comment on the phenomenon of "lost mathematics" in general. Perhaps another light in which to view the issue is that the mathematics at issue has gone to "sleep." Sometimes this occurs because the work is written in a language that does not have many readers. Sometimes the issue is that it was written by a person whose work does not have many "followers" and who did not have a broad context in which to understand what had been accomplished. Finally, often a "thread" of mathematics goes to sleep because with the mathematical tools of the time the line of work involved has gone as far as researchers at that time were able to carry it. When the "sleeping" thread gets reawakened and looked at, sometimes new ideas and tools are available to carry an earlier line of work much further.