Borrowing from Moorcock's The Dancers at the End of Time
A distant future of some race who mastered... everything. Their world was manned by orbiting reality engines capable of restructuring matter, serving the whims of the inhabitants of that world who were schooled in the codes the machines read telepathically from the operators. One only need to know the codes to hack reality.
Time passes. Boredom, bad code, unforeseen and sudden threat... something plucked these godlings from the world.
The most potent machines, the reality engines, go silent. The maintenance engines that regulated climate and safety, no longer registering the inputs of either the reality engines or the masters, go into standby mode with minimum climate control to conserve energy.
The menageries (that included savage men) that were kept private game farms and parks the size of nations if not continents, grow fallow, the animals and men continue to survive as the world enters an ice age, and eventually, emerges from the ice age as the climate engines begin receiving erratic signals that ghost the will of the old masters.
The remnants of the old civilization are all but literally wiped from the face of the planet, scoured by glaciation, the world is changed, and into it, a savage but changed man explores. The minds of some men are growing to duplicate the old masters, but lacking the codes, they cannot access the reality engines with any reliability... only the climate and security engines respond, sensing something, and not needing complex commands, begin operating on standing protocols.
There may be hardware based interface devices or boosters for the reality engines... but the codes are still missing. So trial and error... and error, and trial, and limited success, but the reality engines take thousands, or tens of thousands, of years to master, and the fleeting lives of men barely allow rudimentary influence, even with an ancient artifact, and teaching the codes, the rituals, the words and motions to structure the thought to make the engines understand, change from person to person because no two minds are mapped precisely the same... and so the science of the ancients is a mystery, naught but magic, passed on from gifted master to gifted student, so rarely with success.Those beings able to reach out to the climate and security engines have an easier time... the codes are easier because so much of it runs on protocols... but so much less powerful.
The earth is a warren of tunnels, where all manner of curious construction and machine tunnels where there are still the remnants of the old engines, and rogue engines lurk both in the depths, and in the sky above, touched by erratic minds of men, they have grown some semblance of thought and take peculiar, unpredictable action, anticipating desires of long gone masters in the context of a radically changed world. Those in the depths churn out all manner of strangeness, mimicking the whims of the reclusive masters who choose to claim their demesne as a dark underworld, and not surprisingly, those temperaments, mimicked by corrupted engines, produce horrifying results.