One interesting thing about the age of the Internet is that examples of this are much less common than they ever have been before. There is a glimmer of hope in that many historical accounts of things, instead of being 'lost' as you put it, have simply been buried beneath more popular narratives. Nowadays, a historian can find something new, do something with it, and make the whole world a little more enlightened. To put this in perspective, scientists have understood how stars are actually formed for less than a decade.

Certainly, historical truths can be buried; those scientists spent many a decade believing a different narrative for the formation of stars, and so did the population at large. Although we know the truth today, the Turks denied the Armenian Genocide; the Americans denied the Trail of Tears; the Soviets denied the Holodomor. Those three events seem as evident as the sun in the sky to us today, but if you asked 1840 America what it thought of the 'Indian Removal Act', there might not be so much hostility. It could be said that, for that period of time, the Trail of Tears was 'lost' to history, although I don't think that's the right phrase.