What if there had been almost no surviving records about Ancient Egypt left
I'm not too sure it would be doable, safe a systematical campaign of destruction continuously and consistently made since the end of pharaonic era. Ancient Egyptians simply written down a lot of things during more than 5,000 years of existence, far too much to not let a lot of remains.
This pretty much ammounts to asking about Greek or Romans civilisation being entierly unknown because all of what they left somehow systematically disappeared.
Even civilisation largely unknown before their discoveries, as Hittits, simply let too many traces.
the language completley unknown (Champollion does not decode it)
While Champollion can be credited without problem having genial intuition on this, he was hardly the only one : every linguist with two cents of ambition was on the thing. Sooner or later, someone would have figured it out.
Im thinking of the Egyptian time like the 400 year dark age in Greece where no records were left
But Greek "Dark Ages" didn't destroyed the previous writings, and Mycenean Greece clues can still be used.
As for the Egyptian case, it was simply far more develloped culturally than Mycenean, too important and influent to simply know a "dark age" :
roughly speaking, the disappearance of Linear B in Greece is largely tied with the fall of palatial organisation of Myceneans, when invasions in Egypt generally "fit in the shoes" of pharaonic organisation.
Assuming, tough, hieroglyphic script in spite of being largely used in Egyptian society, simply disappear. We'll end with another script on a same language, a bit like Champollion used coptic and demotic to decode it : you'd just add one layer on the whole thing.
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That said, we could have a more parcellar knowledge of Ancient Egypt, but that would require, IMO, important changes in its history. Roughly, no Middle or New Empire with important acculturation (a wild exemple that I don't consider being viable : Nubianisation on the South, Berberisation on the West, Indo-Europeanisation in the North, etc.).
A good part of Antiquity's Egypt language/culture would be distinct enough (altough the sheer force of acculturation of Ancient Egypt was huge and more than prooved, so I don't think that's even a plausible outcome) that using native languages to decode hieroglyphic script would be a pain.
But at worst, save systematical destruction from omniscient immortal vandals, we'll be more on an Etruscean-like situation : writings are there, we can maybe decipher it, but we can't really say what the hell it's about.