Funny you should ask!
My 4e game started with the end of a great empire as it was overrun by a combined army of goblinoids, orcs, gnolls, ogres, kobolds and lizardfolk held together by a cabal of death knights. This army, the Six-Fingered Hand, crushed civilization, and only one real city survived (thanks to the heroic efforts of the pcs).
The city in question had to undergo some major changes in order to make it- the entire upper district, previously home to the aristocracy, was cleared and converted to rice fields and orchards. They managed to hold out against a prolonged siege and finally the pcs drove off the enemy, sought out the death knight in charge and laid him to rest by exorcising the spirit of his lost beloved.
In the aftermath, the Six-Fingered Hand collapsed into feuding bands of marauding humanoids. Outside of the city that survived (Fandelose), the 'civilized' races were severely depopulated and driven into desperate tribes of refugees barely able to survive. The remnants of the Hand hunt and kill or enslave them, so there really isn't much left outside of the city.
That was about 30 years ago. Since then, the city has struggled to survive, swinging back and forth between civilian government and military dictatorship. Tensions are high. The military desperately wants to keep the people in the city- it just can't protect them outside- and the farmers have lost more and more rights, now being bound to the land. This has led to increasing numbers of protests and the founding of a small, pretty much completely-unsustainable village a dozen miles away that has a great deal of support from the populace but is officially condemned by the army.
The population of Fandelose is slowly diminishing, unable to sustain itself. Much of the empire's culture is gone, many technologies have been lost, most of the empire's gods are forgotten or half-forgotten, and the more organized humanoid bands are still a very real threat.
I don't have any good resources for you to draw on, but there's a good example of a largely depopulated world.
EDIT: For clarity, this is still the setting I'm using in 5e, just a few decades after the siege (those 30 years I mentioned).