Technology.
Technology is another example where it is quite difficult to lose it. Even in a war most people usually survive and the victorious nation/culture/state inherits. There are several ways to lose technology however.
1. It becomes obsolete. Sometimes people get a romantic idea about something but sometimes this explanation is a good one. For example Damascus steel has a certain belief and aura around it and it died out towards the end of the 18th century or early 19th century at the latest. The mines are still there, you can go and get some ore still if you want. Damascus steel was good for its time, notice that it died out when the industrial revolution kicked off? Damascus steel is not magical its not going to cut through concrete so why spend a week or more of your life forging it when mass produced industrial steel will do the same thing? That and the British for example could mass produce this
https://en.wikipedia.org/wiki/Pattern_1796_light_cavalry_sabre
A more modern example would be WW2 equipment. Enthusiasts might love to get their hands on a Tiger tank but they are not made any more for a reason.
2. Loss of resources.
In the modern world we do not really have this problem (yet). In ancient times the Byzantines for example were famous for using Greek Fire. There were actually 4 overlapping technologies here- the Greek fire, the weapon that fired it, the dromon that equipped it and the pump system. Greek fire dies out however and there may have been numerous reasons the loss of access to oil in the middle east was probably a key factor. Another example was Roman concrete. The Byzantines kept a lot of roman technology and could build things like roads and aqueducts after the fall of Rome. Roman concrete however used volcanic materials found in Italy and the Byzantines lost control of Italy and the technology perhaps was lost before they actually lost control of the areas they could get materials from. Concrete was rediscovered later- in Italy.
3. Economic Reasons
This means basically that someone stops paying for the technologies application. This could be due to political or cultural considerations. I have used the example of Roman concrete before and writing and a similar argument applies here. In Roman times the state and rich individuals paid for things like roads, buildings and monuments. With the economic disruptions in the west starting in the 3rd century the Roman economy essentially collapsed and the old families lost their power. Things like aqueducts failing predated the fall of the Roman empire. What probably happened was that the work dried up in the late empire as other problems and less money to deal with those problems took priority. Fixing a public building took a back seat to the proverbial horde of barbarians literally kicking in the door. The western empire lacked the money, the eastern empire lacked the resources. Pozzolana would have been difficult to ship assuming the Italians were still mining it and the Byzantines had problems with Arabic pirates once they lost Egypt and Africa. The Byzantines also did not build monuments on the scale of Rome with a few exceptions such as the Hagia Sophia. Once again money may have been a problem as the Byzantines were smaller than imperial Rome and the loss of the Levant, Egypt and North Africa perhaps represented 2/3rds of their income.
