hi; there have been several interesting threads on this period

• When did Roman rule effectively end in Britain and was there a single, overriding cause?

• Did the Romans simply leave Briton with the collapse of their empire

• How fast was the decline of civilisation in Europe (including Britain) after the fall of the Roman empire?

• Britain, right after the Romans left.

• What was life like in Sub-Roman Britain?

• Why did Britons stop living in urban communities after the fall of the Roman Empire?

• were living standards better in Sub-Roman Britain or Anglo-Saxon England? - 455 AD

• if you'd gone to southern Britain in 460 and asked people "is this still part of the roman empire?" would most of them have said "yes" or "no"?

• What was left of Roman Britain by 700 AD?

I know that /u/Searocksandtrees has given you a wonderfully complete reading list in response to your question, but I'm going to jump in to provide a TL;DR since I wrote my undergraduate thesis on this exact topic this past spring.

There's a frustrating lack of literary evidence from the period which makes a complete narrative almost impossible to construct, but there is an abundance of archaeological evidence that helps us to draw a rough picture of what happened.

Essentially, the "Post-Roman" period in Brittania was marked by a period of increasingly decentralized governance. As the Romans pulled their military backing (and administrative funding!) out of Britain, there was no longer a centralized military force to keep it safe, rendering the town and villa style living that the Britons had used under the Romans so insecure as to be unsustainable.

The local communities in Britain seem to have responded by pulling inwards and banding together in a bid to provide a measure of security for themselves. There is some fascinating evidence to suggest several amphitheaters in Britain were crudely fortified during this period, as the townspeople withdrew to the ampitheatres' immediate vicinity for protection.

Similarly, hillfort settlements (essentially a stronghold at the top of a hill surrounded by a wooden palisade) demonstrate a resurgence in the immediate "Post-Roman" period, further suggesting that people were searching for a local solution to their security problem, even if that meant throwing their lot in with warlords.

This is a really brief overview, but hopefully I was able to shed a bit of light on the subject!