You might be interested in some previous threads about the topic while you are waiting for an answer. It seems it was not so much of a sudden disappearance as a slow change and conversion over centuries. For instance here our u/cleopatra_philopater writes about the decline of the Egyptian language(s) and mentions also the polytheistic religion, noting that both were on the decline before the Arab conquests. And here our u/frogbrooks discusses the Arab/Muslim conquest of the country, how things changed and whether it could be considered colonialism. There is also this thread where our u/khowaga takes a broad look at Egyptian history when asked about genetics and ethnicity. Finally, we have this recent thread by u/Necessary_Cry_5301 and this older one by u/daretelyam about modern Egyptian identity.