It doesn't matter.
The civilization fell. The inhabitants forgot everything. It is now irrelevant why and how exactly it fell; it is irrelevant because nobody knows, so the specifics of the decline and fall are now lost in the mist of time.
Examples:
There was once a great power in the Levant, which endured for thousands and thousands of years. But eventually it fell on hard times, and it became easy to conquer, and in due course it was conquered by the Persians, who were later conquered by the Greeks, who were later conquered by the Romans, who were later conquered by the Arabs, who were later conquered by the Turks.
When it came to pass that the French invaded the country and defeated the Turks, they found that the inhabitants had forgotten everything about the rich and resplendant civilization which had once flourished there. They had forgotten its name, they had forgotten its language, they had forgotten the great victories and the names of the glorious rulers. The impoverished peasants who inhabited the land called it Misr, using the name given by the Arabs; at least the French used an older name, Aigyptos, Egypt, bestowed by the Greeks two thousand years earlier. The original name, Kemet, was found out only in the 20th century.
There was once a great civilization in Asia Minor, who rivalled the power of Egypt, and which endured many centuries. But eventually it fell, and its lands changed hands from conqueror to conqueror. When its royal archive was discovered accidentally in the 19th century by a German scholar, nothing was known about its former glory. Nothing. Not its name, not its language, not its history. The German scholar calleed it "Hittite", using the name of a mysterious nation mentioned in passing in the sacred books of the Hebrews and which had appeared from time to time in ancient inscriptions.
There was once a rich and vibrant trading civilization in and around the Egean, with advanced architecture, and writing, and art; it endured for a long time, but eventually it fell, and while it was not completely forgotten all the details of its fall were lost. People only remembered, vaguely, that it had existed, and half-remembered the names of its former cities and kings.
What all these have in common is that their downfall was caused by one event, of which we know almost nothing. We call it the Late Bronze Age Collapse, we know that is was sudden, brutal and definitive, and we know that it happened at some point between 1200 and 1150 BCE. In the words of the American historian Robert Drews as quoted by Wikipedia:
Within a period of forty to fifty years at the end of the thirteenth and the beginning of the twelfth century almost every significant city in the eastern Mediterranean world was destroyed, many of them never to be occupied again.
And that's about all we know about an event which ended several major civilizations here on Earth, an event which was of the utmost importance in the eventual rise of our own civilization on their ruins. We have several fragmentary indications that what we call the Late Bronze Age Collapse was in some way linked to an invasion, or a migration, by a group of people whom the ancients called the Sea Peoples, but we don't know who they were and were they came from, and actually we are not even certain if they were conquerors or displaced refugees pushed by desperation.
In fiction, this device is used for example in S. M. Stirling's series The General. The series follows the adventures of Raj Whitehall, a general in the service of the Civil Government, who attempts to re-unite the half-barbarian kingdoms of Bellevue, a planet which had been once a member of a star-spanning Terran Federation. The most civilized of the inhabitants sort-of remember that the Federation had once existed, but they know nothing about its nature or why it fell. As a sop to the readers, the sole surviving A.I. -- who is helping Raj in his efforts -- mentions (halfway through the fourth book in the series) that the Federation fell because something called the Tanaki Spatial Displacement Network was destroyed, or maybe it was shut down, and in any case all contact was lost with the rest of the Federation, and eventually a civil war ensued, and very powerful, possibly nuclear, weapons were used.
And that's it: nothing more is said, because nothing more would be relevant. There was once a Terran Federation, and it is no more; its technology has been lost, its language has been lost, its culture has been lost. The people living in the times when the story is set know that they are living in fallen times and not much more, so no purpose would be served by providing more details.
