The Greeks and the Romans wrote quite a bit about Egypt, but most of what they saw was the tail-end of the Pharaonic era. Herodotus in particular wrote a famous "travelogue," but some scholars don't really trust what he wrote (or even believe that he was there). Even so, such writings at least made people generally aware of the basic concept.

Coptic, which is essentially Ancient Egyptian written with Greek letters by Egyptian Christians, was pretty common for a while, but any records written in Coptic were from ~400AD and not exactly ancient in Egyptian terms. It did give scholars a general idea of what Egyptian sounded like though, which was instrumental in translating the Rosetta stone.

The last source of information was of course archaeology, which probably told us quite a bit about the general sweep of Egyptian history.

Before Rosetta, we did know the names of a number of Pharaohs and when they reigned, but there was no solid chronology and the further back in time we went, the more muddled things were. That's still the case, but now our point of utter muddlement is further back.