Another brief post, but in the same tent (roughly) as the Trojan War piece from a few weeks ago.
I have been digging into Dan Simmons's Ilium of late, rereading in between other works. His work either is tends to either detailed historical horror (IE, The Terror, Black Hills) or far-future space opera with heavy reference to literature (Ilium/Olympos, but more prominently, Hyperion).
Anyway, one strand in Ilium/Olympos concerns the folk ('old-style humans') on a far-future earth who live century-long life spans with rejuvenation every twenty years. Their lives are idle and hedonistic, all necessary tasks for survival being carried out by biological robots ('the Voynix'). Teleportation nodes provide for all transport. Fatherhood is basically unknown, thanks to some tampering allowing for sperm storage - though women are only permitted one child. Said old-style humans can't even read.
They are set in a world where familiar features have warped and changed. Paris (population 25,000 - and considered a metropolis) has a vast crater at its centre and is overshadowed not be the Eiffel Tower but by a vast statue of a naked woman - semi-transparent and filled with a photo-luminescent red liquid (a 'Lost Age artefact'). For some reason, the Golden Gate bridge is now by Macchu Picchu. In the unpopulated areas between teleportation nodes of the 'faxnet', resurrected species lurk (smilodons, allosaurs and more). Folk killed by them, can be resurrected - if close enough to a node.
Functions similar to a smartphone, if more advanced, have been implanted into the residents of the future - but are taken so as part of nature that most of these functions have been forgotten. This, and most of the state of being enjoyed by folk in this time are the workings of the Post-Humans dwelling in a vast orbital ring (hence 'Old-style humans).
These are the remnants of humanity, dwelling in the structure of a forgotten world, lacking the skills or curiosity to explore or develop much; lightly and subtly governed by those who have gone before.
This is merely the setting. Then everything falls apart: the teleport ceases, the voynix turn nasty and things collapse - and no-one knows how to cope, miserable eloi that they are.
Think The Culture - but no Contact, no Special Circumstances - and then all the Minds vanish. And everyone is useless.
Naturally, the protagonists are the exception - or at any rate, get a head start on a vicious learning curve. Assisted by Odysseus, son of Laertes and a televison-equivalent viewing of the Trojan War (I won't explain that bit here, but this is the Classical connection).
I think this strand of Ilium has potential: quite a few post-apocalyptic settings have humanity in the ruins of the old world - but tend to have some form of community come together already by the start of the story. Likewise, there are a variety of settings where poorly understood technology looms over all (generation ship stories like Orphans of the Sky or Non-Stop do this), though the fruitless hedonism angle rather calls to mind Logan's Run. Perhaps more recent and relevant is the strangely named Horizon Zero Dawn, summarised and discussed in tabletop terms at Throne of Salt: the mechanical beasts that imitate prehistory especially.
But of course, the tribes of Horizon Zero Dawn or the monks of A Canticle for Liebowitz are fully developed by the start of the narrative. No-one has to work out hunting and gathering from scratch, let alone forging anti-voynix weapons. One could perhaps look to Kingdom Death Monster and its settlement and technology mechanics - scaled as appropriate (not to mention the extensive reskin!).
This feels quite current, in its way. Your technology has failed you now; you can't fix it and you didn't make it and now all your skills are useless (You used to work out? Super set of abs, but can you kill a charging aurochs? You used to garden? Lovely tulips, what do you know about crop rotation?). Humankind didn't just pull the ladder up after itself: they burnt the drokking ladder! An immediate switch from garden parties to fighting tyrannosaurs, with the accompanying learning curve.
Throwing in a visit from Odysseus (or similar) would be a way to soften things - but this is taking a catapult into Fantasy RPG land very quickly. Being able to read is nigh-on magic; being able to read and understand even more difficult: Old-Style humans probably don't even have a system of measurements. At the tabletop one could factor this in - using non-inidcative or nonsense names for real-world concepts (A mile is a faga; a hard drive is a blarter) in in-universe texts. Translation could evolve slowly.
You may wish to read Ilium and its sequel Olympos first. Don't ask me where the Proust-reading Jovian robots come into all the above, however.