Here’s a brief but very good article about the collapse of Britain in the fifth century, after Rome was obliged to withdraw in order to defend the home front. The resulting post-Roman “recession” lasted for 600-700 years. Why?
The reason the Romano-British economy collapsed so dramatically should give us pause for thought. Almost certainly the suddenness and the catastrophic scale of the crash were caused by the levels of sophistication and specialisation reached by the economy in Roman times.
The Romano-British population had grown used to buying their pottery, nails, and other basic goods from specialist producers, based often many miles away, and these producers in their turn relied on widespread markets to sustain their specialised production. When insecurity came in the fifth century, this impressive house of cards collapsed, leaving a population without the goods they wanted and without the skills and infrastructure needed to produce them locally. It took centuries to reconstruct networks of specialisation and exchange comparable to those of the Roman period.