Well, let's break down some possibilities.
First, the loss of technology could be either intentional or unintentional.
Intentional
Intentional loss of technology essentially means that the population willingly decided to discard some or all of the technological advancement they had gained in one, some, or all areas. Reasons for this might include:
Religion
Religions can pop up that decry the use of certain technologies, and if those religions take over a populous, there is an inquisition.
War
The uses of certain technologies in war can cause them to be heavily limited or outright banned once the war is over due to the terrors that that technology wrought.
Public Mistrust
Sometimes a technology is sketchy enough in its own right that a societal shift is enough to cause it to be generally seen as inherently evil or untrustworthy. Technologies common in this category might include things like poisons/alchemy, necromancy, cloning, or cybernetic augmentation.
Public Disinterest
Opposite the previous, there's also the possibility that a technology is just so outdated or mundane that interest in it wanes to the point of extinction. (This is possibly what happened to the original techniques of producing Damascus steel.)
Unintentional
Unintentional loss of technology stems from things outside of societal control, such as:
Resources
A widespread technology might suddenly find itself unusable due to the depletion of the natural resources it depends on. At this point, the society that uses it has to either pivot or regress.
War (Again)
War can cause unintentional loss of technology, too. For example, when a society which is the only possessor of an advanced technology is eradicated by a neighboring country that had declared war for unrelated reasons.
Economic Strife
Advancements in technologies that improve comfort and reduce inconvenience tend to be put aside during times of hardship where the primary goal is to just survive, and of the times last long enough, those technologies might become forgotten.
Natural Disaster
Similarly to war, a natural disaster can cause the downfall of a society that had enjoyed advanced technological development, causing those technologies to be lost.
This list isn't exhaustive, and each of the sub-categories can have sub-categories of its own. You don't necessarily have to pick just one category either; combining multiple categories can give rise to a scenario on its own.