Maya Glyphs
The Spanish nearly wiped off the rich Maya culture in the sixteenth century. The cultures of the survivors were destroyed. Their language was replaced by Spanish, and their beliefs were superseded by Christianity. It's the same scenario as any other time in history when a powerful conqueror arrives and decimates the native population; the devastating loss of culture and heritage.
Thankfully, part of the Maya culture has persisted, including the glyphs. Since the beginning of the conquering of their culture, the almost-lost language has been researched. One of the four illustrated Maya books that are still in existence are thought to have been sent back to Spain by Cortes.
The Maya glyphs proved to be difficult to decipher. With these, the Spanish had little success. The 1800s saw a slight increase in efforts, but it wasn't until the 1900s that many of them were entirely translated. When the language could be spoken aloud for the first time in several hundred years in 1981, the final piece of the puzzle was revealed.