> Two quick Qs:
> * When did an entire civilisation 'forget' how to
> speak, read and write to their children?
There are no quick answers.
If you consider the nature of the languages it's much easier to understand how this occurred; spoiler, it didn't happen suddenly but was a process that played out over 1200 years (really it was likely more like 2000 probably).
Ancient Language was very simple except that it was metaphysical; it contained all human knowledge and was the means to acquire it. No, not like a dictionary contains all literature but rather their words were the knowledge and the grammar was the rules. As more and more knowledge was added it became harder and harder for people to understand. By 3200 BC it was complex enough that the slower people had great difficulty expressing themselves or understanding. A sort of pidgin form of the language arose that employed the same vocabulary but words were symbolic rather than representative and meaning was simply stated. This was open to great confusion so writing was invented to stop drift in retelling.
This worked well for centuries as the language became ever more complex and more people dropped out of using the Ancient Language. It was very difficult to go back and forth between these languages because they were incompatible. Translation is impossible and it was very difficult for even the brightest modern language and Ancient Language speakers to communicate. You need to switch gears to understand each other. By 2000 BC there weren't enough Ancient Language speakers let to operate the state and its institutions. There was simply a law passed changing the official language to modern language and this is confused as the story of the Tower of Babel. Science continued for at least several centuries but the scientists couldn't communicate with others and eventually they just died out.
Modern language speakers never forgot how to speak Ancient Language; most of them never knew. They were born speaking Ancient Language but their parents couldn't reinforce this so they had to learn modern language. Modern language isn't necessarily more difficult to acquire but we must each forget the one we babel as babies.
Ancient knowledge can't be formatted into modern language because meaning is open to interpretation in modern language and reality is never open to interpretation. This means ancient science died with Ancient Language.
The only thing that saved the stinky footed progeny was that ancient technology was quite simple and could be passed down father to son and though apprenticeship. These first modern speaking bumpkins knew their ancestors were powerful and wise but didn't know how or why. They had no science at all and had only the ancient technology to keep them alive.
> * What might have caused one generation of an
> entire civilisation to fail to understand anything
> spoken or written by that of their parents'
Initially the bumpkins did understand a great deal about such things like how the pyramids were built. Many people studied the old writing seeking the meaning but they lacked the scientific understanding to succeed. They could pass on what they did know and they did. This is why history finally starts in 2000 BC. It was the Tower of Babel that began history because for the first time things were recorded in language we could understand. The people of the time emulated and venerated their ancestors but it was impossible to understand them. The writing was lost because almost none was copied. Why copy gobbledty gook? There was still some left when the Greeks arrived but they had little more luck understanding it than the earlier Egyptians. The Greeks "consumed" this writing by the same processes until nothing was left. It was the 1880's that the first corpus of sufficient size to solve appeared in the form of the Pyramid Texts. The problem was that the 1880's barely had the scientific resources to solve them either and without high speed communications there was no way to work on it. Even as recently as 1990 the search engines weren't up to the task. At every stage there are good reasons it wasn't seen and wasn't done. By 2006 when I began (back when the search engines still worked) nobody was really looking. Everyone believed the meaning of the PT was set in stone and the Egyptologists had a handle on it. Few people doubted that humans were once superstitious but we're all better now.
You see what you expect but even more true is you don't see if you don't look and people weren't looking.