Answer:
If their writing dissapears, then, what is known about that civilization is also likely to dissapear, unless their previous writing was preserved in some other way.
This is something that has actually happened often in history. For example, the library of Alexandria, in Egypt, was the largest library of the Ancient World, and it was burned down by Caliph Omar in 642 AD. Countless works by Ancient authors, that gave account of civilizations, cultures, philosphies, and religions, were lost, and there is no plausible way to recover such knowledge in the current era.
