Lost technology

Not to be confused with alien technology which shares some similarities with this.

Variants:
* The technology is (revered and) kept unused because people fear of losing even that. Brought out only when nothing else seems to work. Implies people don't know how to maintain or create more and somehow have failed or refused to reverse engineer it (e.g. its workings far exceeds the knowledge that still exists on the required fields).
* Significant parts of the technology can't be reproduced, either due to lack of of knowledge or manufacturing capabilities. People still can maintain and reproduce most of it, though that Important Bit is irreplaceable. Safeguarded by those who have it and likely avoided use. Often that small bit finds new uses though, though what those uses are depends on who is making them. Commonly a power source of some sort.
* Technology is somehow indestructible and never runs out of whatever powers it, thus is ends up being used whenever possible. People often have no hope of re-creating the technology tho, so it is valued beyond all else.

Often attributed for the destruction that caused the loss of the knowledge/manufacturing capabilities of the technology itself, thus possibly avoided because it's "cursed" regardless if that's true (likely just misused/abused by the creators).

In all cases the number of lost technology never increases, thus arming everyone with lost tech is unlikely unless vaults or other storages of it can be found. And even if the actual tools can be created, the lost tech part of them remains missing and work mostly as replacement parts in case the others break.

Reverse engineering often (but not always) has catastrophic consequences.