They became what you now know as the "elites"

>>16812112
Something like 60% of the population was enslaved or killed indiscriminately, so, yeah.
We know that Contantine XI's brother became a very high ranking official in the Ottoman Empire so there's something you can infer from

>>16812112
>Fled to Western Europe (most of them)
>Enslaved or killed
>Live as second class citizens
>Somehow be lucky or bootlicking enough to be integrated into their new order

>>16812112
>>16812140 This is largely accurate though I'd like to expand on it.
You can divide what the Byzantine bureaucracy and aristocracy did into four broad classes.
>Emigres
The first left for abroad, often but not always with great fortunes, and would typically find themselves as officers in mercenary companies as stratioti, serving mainly in the Italian states but could be found all over Europe, in the HRE, France, Spain, England, and some lesser known ones in Eastern Europe too. These are also the ones who contributed to starting the spread of art, science, governance, and philosophy, long held within the Byzantine Empire outside of it. As an aside, I find it interesting how depending on where they went, different facets of Byzantine culture were absorbed. In Italy, with numerous states and different means of governance, you had the artistic movement of the Renaissance, taking away the strict adherance to traditions of art and freeing them to become more humanistic. Whereas in the East, Orthodoxy and autocracy were doubled down on in Russia to levels unimaginable in Byzantium. Some of these would eventually return to Greece from the 1830s onwards but not all.

>>16812260
>Serving the new masters
The second group would ingratiate themselves with new authorities, be they Ottoman or Venetian. They would largely run the affairs of their communities and be used in tandem with the Church to keep things calm. They largely carved out wealth for themselves as a mechant class. Think of the Phanariotes or Greeks in the Venetian Libro d'Oro. Until the Greek Enlightenment they were largely content and largely reviled, but afterwards the community was divided between the pro-reform Eastern Party who sought to elevate the Greek/Rum community within the Ottoman Empire (think what Austria-Hungary would later do), and the pro-independence Western Party. Those in the Western Party greatly contributed to the Greek Revolution, and led to the downfall of Greeks remaining in the Ottoman Empire.
I'd argue a subset could be included who converted to Islam but this was considered 'turning Turk' and thereby distancing one's self from the Rum millet, although Crypto-Christians did exist too.
>Klephts
The third group devolved from nobles into bandit lords and pirates, who could occasionally be bought off or bribed, only to then turn on the Turks with weapons bought with their own bribes. Maniots, Roumeliotes, Arvanites, Souliots, Cretans, and a lot of Aegean Islanders did this. During the Revolution, they contributed greatly, but would also undermine it by being so unruly and feuding. Some would occasionally leave like the first group too if Ottoman pressure became too hard. The Coronian community in Messina and Maniots in Corsica are examples who did both these
This group really contributed to the Greek/Balkan cultural attitude of disrespecting political institutions since for centuries it was synonymous with patriotism.
>Put to death
And finally, some were put to death, root and stem for resisting.