The example that first comes to mind would be the collapse of the western Roman civilization. Estimates of the population of Rome itself, or of the empire in general, have been debated, but there were roughly a million people at the time of Augustus (around A. D. 1), and was in the neighborhood even in the late 300s, but falling off a cliff in the 400s to maybe half of that. Some cities were even lost, like London.

In "The decline of the Roman Empire in its western provinces precipitated a general trend of deurbanization. Are there examples of cities in W. Europe (5th-6th cent. C.E.) that actually grew during this time?" at https://www.reddit.com/r/AskHistorians/comments/29f6x4/the_decline_of_the_roman_empire_in_its_western/ , /u/bitparity (flaired for Post-Roman Transformation) wrote, "In Western Europe, of cities that had existed during the empire during the 4th century, which grew in population during the 5th and 6th centuries: none, notta, zippo.". They and /u/Tiako discussed a bit about causes, one suggesting that Syracuse probably was least affected, and maybe Paris because it was a later capital.