In OTL Giuseppe Garibaldi unified Italy from a region of different states with a common language (including many very different dialects) to a single country. What would have happened if he died before he could make any progress in unifying Italy?
The short answer is this: Someone else would have done it.
I know this isn't the answer you're looking for, but something like unification has a lot of social momentum. Maybe it would have taken longer but the same thing happened in every country across Europe sooner or later. It was not a one man operation, it takes a lot of people working together to make disparate city states into one country. This was a thing happening, it was unlikely that any one person could have stopped it rather than saying one man did it.
People had been calling for unification since the 15th Century, Machiavelli called for it, the Medicis supported it. The Hapsburgs unified Spain in the 16th C, England unified in the 11th C. Germany unified around the same time as Italy (1871).
Expanding on user16295's answer, some form of unification probably would have happened because of convergence of multiple social, economic and technological factors, but the end result might not resemble Italy as we know it today (the same may be said of other European nations).
Given the cultural and economic differences between northern and southern Italy, we might have seen two separate kingdoms arising where Italy is today. A "Northern independence movement" actually exists in the form of the "Northern League", an Italian political movement which advocates separating the wealthy north from the south (among other things. It is also a Euroskeptic party and advocates withdrawing from the EU as well).
Another factor that wasn't at play in places like Spain or Germany is the existence of Rome as the seat of power for the Catholic Church. The Church was a major political power in Italy from the fall of the Roman Empire onwards, and a politically astute and active Pope or series of Popes could have derailed the project, or united Italy under the political leadership of the Church, making Italy synonymous with the Papal State. Even without such activist Popes, it is quite possible the Papal State could have ended up being much larger and temporally more powerful than it is in OTL.
Combining these factors might see the Italian peninsula divided between several principalities, a "Northern Italy" dominated by the cities of the Renaissance (with perhaps Milan as the capital), the Papal State centred on Rome and a poor and rural Southern Italy. Although the "Serenìsima Repùblica Vèneta" had been extinguished by Napoleon, it is quite possible that that city state might have re emerged in the chaotic conditions given the lack of a centralizing force if a fractured Italy was created.