For example, when the Britons began relocating to Brittany (to escape Anglo-Saxon barbarians then invading their homeland) they discovered that there were no people living there although this had been one of the most populated, progressive and productive regions in the Western Roman Empire.
This was a common situation throughout Northern and Western Europe. Note that the Dark Ages begin about 538! Some claim they began precisely that year, or in 535. We really don't know because they lost track of their calender at that time!
There was massive loss of records. In fact, there was massive loss of everything. Life was never the same after that time.
Did you imagine that people just forgot how to do stuff and retreated to a neolithic lifestyle?
People died in large numbers. The survivors faced a far less populated world. It took Europe half a millenium to begin to recover!