The Holy Order of Engineers

You can take this more or less literal (keep the religious aspect or divorce this answer from it), but either way it works.

Due to an internal power struggle long ago (that possibly developed into a civil war that was stamped out), the Powers That Be determined that equipping the masses with the understanding of how to build or develop weapons of war or other technology that could be leveraged militarily, economically, or politically were dangerous to have in the hands of the masses.

And so the Holy Order of Engineers was created. This very closed-mouth and tightly-knit organization was given authority and charge over all higher learning, research, and development that possibly could relate to engineering or R&D, or in many cases, the actual manufacturing (for the technological crown jewels). The Followers of Engineering Ongoing was founded as a sister organization to the Order that was taught how to maintain infrastructure. The Followers were overseen and governed at arm's length by the Order as while the Followers were less dangerous than the Order, they still had the potential of arming the masses with easily exploitable technology. The Order dispatches Inquisitors routinely not only to weed out engineering from the masses, but to check in on the Followers to ensure that anyone too intelligent, creative, ambitious, or otherwise on the path of engineering was either recruited or put to the sword (or mag rail gun, as the case may be) and made an example of.

And in this manner your civilization continued for quite some time. Centuries perhaps.

But like all great things this came to an end. The governing authorities grew to not trust the Order and their ever increasing influence, or perhaps the Order grew too proud and attempted to rebel. Either way, while the Order understood amazing technologies, they lacked the infrastructure and manpower to defend them. It wasn't long before the Order was all but stamped out, and the last few members of the Order followed their holy code to protect their archives from the masses, and deleted or burned them all. (If in the modern era, the Order probably had designed a magnificent virus that was able to crawl through all their servers and scramble the data irreversibly)

The last few members of the Order defected and were folded into the Followers, however these measly few weren't enough to provide understanding of these incredibly advanced technologies besides the insight to deal with perhaps more advacance troubleshooting, and even then not in all cases.

And so your civilization limps along, possessing the marvelous technology of the Order, but none of its understanding.

The Holy Order of Engineers

You can take this more or less literal (keep the religious aspect or divorce this answer from it), but either way it works.

Due to an internal power struggle long ago (that possibly developed into a civil war that was stamped out), the Powers That Be determined that equipping the masses with the understanding of how to build or develop weapons of war or other technology that could be leveraged militarily, economically, or politically were dangerous to have in the hands of the masses.

And so the Holy Order of Engineers was created. This very closed-mouth and tightly-knit organization was given authority and charge over all higher learning, research, and development that possibly could relate to engineering or R&D, or in many cases, the actual manufacturing (for the technological crown jewels). The Followers of Engineering Ongoing was founded as a sister organization to the Order that was taught how to maintain infrastructure. The Followers were overseen and governed at arm's length by the Order as while the Followers were less dangerous than the Order, they still had the potential of arming the masses with easily exploitable technology. The Order dispatches Inquisitors routinely not only to weed out engineering from the masses, but to check in on the Followers to ensure that anyone too intelligent, creative, ambitious, or otherwise on the path of engineering was either recruited or put to the sword (or mag rail gun, as the case may be) and made an example of.

And in this manner your civilization continued for quite some time. Centuries perhaps.

But like all great things this came to an end. The governing authorities grew to not trust the Order and their ever increasing influence, or perhaps the Order grew too proud and attempted to rebel. Either way, while the Order understood amazing technologies, they lacked the infrastructure and manpower to defend them. It wasn't long before the Order was all but stamped out, and the last few members of the Order followed their holy code to protect their archives from the masses, and deleted or burned them all. (If in the modern era, the Order probably had designed a magnificent virus that was able to crawl through all their servers and scramble the data irreversibly)

The last few members of the Order defected and were folded into the Followers, however these measly few weren't enough to provide understanding of these incredibly advanced technologies besides the insight to deal with perhaps more advanced troubleshooting, and even then not in all cases.

And so your civilization limps along, possessing the marvelous technology of the Order, but none of its understanding.