Answer adapted from this other similar but fundamentally different question

By the year 476 the Western portion of the Roman Empire had lost lost Gaul (to the Franks) Iberia (to the Visigoths) and Africa (to the Vandals). Only Italy and Dalmatia continues to be ruled by Rome. Years of civil wars an infighting has decimated the army, and a long succession of army-backed strongmen had taken turns ruling the empire, many of whom had mixed heritage; Romulus Augustulus, the last emperor, was himself was a puppet of his father Orestes, who was the Magister Militum (head of the army) and although he had married into a Roman family, he himself was not an ethnic Roman (and to be pedantic, the Eastern Emperor never even recognized Augustulus, which allowed Julius Nepos, the pervious emperor, to rule in semi-autonomy in Dalmatia claiming all the while to be Emperor of the West. This can give you an idea as to how chaotic things had been getting in Italy at this point).

Flavius Odoacer, the de facto leader of the Foederati or ethnic "Barbarians" fighting for Rome (who at this point in time made up most of the army), deposed Romulus Augustulus when he lost the army's favor. Odoacer, unlike his predecessors, openly admitted that although his martial law allowed him to rule Italy, he was powerless beyond the alps, and never called himself Emperor, rather, he sent the imperial paraphernalia to Constantinople and asked to be recognized as "Consul", keeping all of the functions of the Roman administration in Italy intact (indeed, Flavius Odoacer as was so romanized historians aren't even sure which "Barbarian" ethnic group he belonged to). He initially played lip service to the pervious emperor Julius Nepos, who all the while had been hanging out in Dalmatia since Romolus Augustulus deposed him. Odoacer even minted coinage in Julius Nepos' name, but this didn't stop him from from (probably, there's only circumstantial evidence) getting involved in the plot whereby he was eventually murdered. Regardless, after the murder of Julius Nepos, Odoacer asked to be given the title of Patrician by the Eastern Emperor, and be recognized as the ruler of Italy under the wing of a single Roman Emperor in Costantinople.

While this was happening, the Eastern Emperor Zeno was much too engrossed in his own issues in Costantinople to give much thought to the ungovernable and chaotic west (again, probably; it's not like we have a convenient letter where he explicitly states what he thought of Odoacer taking power). Zeno had to consolidate his dubious claim to the throne; as an Istaurian, from southern Anatolia, he had few longstanding friends in Constantinople, and he had gained the previous emperor Leo's trust (and his daughter's hand in marriage) thanks to his political maneuvering (read: he tattled on the right people during the right conspiracy). Most importantly, Zeno wasn't much of a military man, participating in expeditions in Greece equal parts vague and minor save for the usual political conniving and betrayals. So hardly had he gained power that internal armies came to a head thanks to two Ostrogoths, both confusingly named Theodoric.

Although the Ostrogoths had been given lands in Pannonia in exchange for the defense of the upper Danube, Zeno came to the conclusion that the Ostrogoths were more trouble than they were worth (fighting with him, fighting with each other, and in a general sense just being another annoying thing to have to deal with) so he convinced them to go conquer Italy. This way, they would stop annoying him, and he could also get rid of Odoacer in Italy, at this point another potential opponent.

So as far as Zeno is concerned, his reaction was "I have had it with all this BS; annoying problem #1, go fix annoying problem #2".

So Theodoric the Amal, King of the Ostrogoths, raised in Constantinople, went to conquer Italy. His people were largely Romanized, having lived within the Eastern Empire's borders for centuries. When he defeated Odoacer and set up the first Kingdom of Italy, he did his utmost to further maintain the Roman institutions, law, and judicial system. However, he did not rule from Rome, rather, he ruled from Ravenna, seaside city conveniently defended by swampy surroundings, and easily supplied by ship.

So here we are, soon after the "Fall" of Rome, the first Kingdom of Italy was established as a client state of Costantinople, and lasted into the sixth century. One political system was substituted for another, and this new system was arguably better suited to the militarized necessities of the time. Relations with Constantinople would weaken and strengthen (mostly weaken) through the years, but there would be no "Shock" that would warrant some sort of easily identifiable unique reaction, as infighting among the Italian ruling class continued much as it did when the ruler of Italy was calling himself "Imperator" instead of "Rex".

The kingdom's fall started in 535, when the Byzantine Emperor Justinian attacked the Kingdom of Italy, taking advantage of infighting (surprised?) among the Romano-Gothic ruling class, deposing the last king of Italy in 553 and consecrating control over Italy (despite towards the end, the Kingdom of Italy had rallied around Totilla, charismatic, fair, just and politically able king. Unfortunately, Totilla was slain in battle, and his reforms did too little, too late).

And then the Byzantine Empire, thanks to its chronic manpower problem, lost Italy in less than a generation. Although Justinian was definitely a great wartime politician (who was particularly good at picking the right man for the right job), his conquests stretched Byzantium's resources too far, and even sympathetic chroniclers criticize him for being unable to defend what he conquered. So I guess the fall of Italy after Justinian could be characterized as a component of the wider, "Darn it Justinian, did you really think this through?" reaction.

Thus, the sparsely defended Italian lands were easy prey for one of the last great "Barbarian" migrations: that of the Lombards. The question, "where did the Lombards come from" is covered in Jorg Jarnut's History of the Lombards (Enaudi, Torino, 2002) but that's a story for another post. The Lombards would contest Italy with the Byzantines from the 6th to the 8th centuries, and by this point in time, the city of Rome was almost a semi-autonomous military protectorate (Run by that one Pope guy). The Lombards moved into the Northern Italian Plains with relative ease, while the Byzantines would hold onto the Pentapolis or five cities along the eastern coast of Italy near the mouth of the River Po (defended by swamps and easily supplied by the sea), the city of Rome and its surroundings, and the easily-supplied Calabria and Puglia regions of Southern Italy.

The struggle would last for centuries. The Lombards eventually "Went Native", imitating the Roman Imperial customs, establishing Mediolanum (modern Milan) as their capital, while still dividing Italy into various feudal subdivisions; indeed, had things gone differently, perhaps Italy would have survived as a medieval state, and centralized as the rest of europe did over the course of the renaissance. But as those bizarre historical turns of events would have it, the Kingdom of Italy would again be dismantled just when the Lombard King Desiderious (d. 786) got very close to finally kicking the Byzantines out of Italy. A successful offensive saw the encirclement of the Pentapolis (that to this day are called "Romagna" since the Byzantines were the last "Romans" to leave) which allowed Desiderious to march south against Rome unopposed. Seeing the Byzantine army cornered in the Pentapolis, the Pope feared a loss of independence for the Bishop of Rome (again, that Pope guy), who had been pretty much guaranteed a free hand to rule the "Duchy of Rome" (modern Lazio) in exchange for nominal Byzantine rule and military access. So Pope Adrian cried "Uncle", excommunicated Desiderius, and asked Charlemagne to come to his aid. Charlemagne, even though he was busy conquering what would later become Germany, was immediately flattered and marched for Italy.

What we can take away from all this is simply that the de-Romanization of Italy happened so gradually (if it happened at all), and across so many fronts (political, social, and military) that there was no single "Fall" that would elicit one single shock reaction as if the United States of America were to lose half its territory tomorrow morning.