Lost tech in China is pretty fascinating. There's archeological evidence of very advanced clocks that predate similar European ones, but they seem to have lost the knowledge on how to build it with the builder (City was conquered, clock was dismantled, his son was unable to reconstruct it). Meanwhile euros were able to push their knowledge forward more or less uninterrupted from the middle ages on.
Seems pretty clear to me that we could easily lose a bunch of knowledge again. Could even argue we're very close.