Novels: Ringworld series.

The ringworld itself is an alien megastructure - a ring the diameter of a planetary orbit, rotating around its own star, designed with a habitable terrestrial environment on the inside surface of the ring.

The ringworld is extremely old. The original creator species is nowhere to be found, but through some backstory it becomes apparent that they are the ancestors of some of the different species that currently occupy the ringworld. Their culture has fragmented in the massive amount of time since the ringworld was constructed. Most of the descendant species no longer have records of the original species they evolved from and no longer even understand the true scale or shape of the world they're living on. Most of their electronic devices have broken down due to a specially engineered plague that targets electronic circuitry.

If you've heard of or played the Mass Effect series, it has a story based around anchient technology. If you haven't played it, you should. The gameplay is good, but the story is amazing.

The Ratakan Infinite Empire is pretty interesting in Star Wars. They're the earliest 'known' galactic empire in the universes's setting. They had darkside technology which corrupted their already violent civilization. Eventually they were seriously damaged by a plague - thought to be engineered by a slave-race, which killed alot of them and cut the ones who survived off from the force, which meant they couldn't control their technology anymore.

Since the Ratakan's were genetically similar to each other, (not sure if they were clones or just highly genetically engineered at some point), the plague effected most every individual. Without the use of their technology, they were defenseless against their slaves, who rose up against them. Unfortunately, now i think the Ratakan are part of the "legends" so not 'official' cannon anymore.

I'd say my personal favorite is the Void trilogy by Peter F Hamilton.

The series includes many instances of Clarke's Law where extremely advanced technologies not fully understood by humans take on a sort of mystical quality. Examples include the Void itself and the Paths used by the Silfen race. Others such as the High Angel colony and the SI are inscrutable but less "magical".

It's a very long read, especially if you also throw in Pandora's Star and Judas Unchained, which take place earlier in the time line of the same universe Still, they're worth it in my opinion. In fact, now that I think about it, the cyberpunk story I'm working on now is influenced by them.

I'll avoid spoilers for now, but could provide additional information.