Any given civilization is unlikely to "forget" how to manufacture technology, nor all of it, even if they did.
A computer is not the same as an abacus, despite both being essentially "counting machines". A lawnmower is not the same as scissors, despite them both being "cutting machines".
If you look at our own civilization, we could, in theory, lose the ability to produce and use digital products, as few people even today (as a percentage) understand how they work, at a fundamental level. But at that same fundamental level, electronic components are extremely simple, as is electricity.
Once a civilization reaches a certain threshold, and the tech of that threshold becomes widely available and understood, it is very difficult to lose that knowledge collectively.
For example, it is highly unlikely that the people of the world today would ever lose the collective knowledge to exploit steam power, and thus electrical lighting. From there we could re-make certain intuitive leaps and re-invent internal combustion, and from there to nuclear fission, as they are all based on the same principles of pressure, heat, and releasing energy from matter.
What would actually likely happen is that a civilization would be knocked back an age or two: i.e. Digital Age to Space Age, or Industrial (losing computers, polymers, plastics and robotics, nuclear power, Modern architecture etc). Industrial to Post Dark ages (following some religious anti-technological sin fever).
The greater the gap between age thresholds, the more knowledge would be lost, and likewise, the greater the catastrophe would be required to bring about the fall. Indeed, knocking a civ down by more than a single age would require something singular and devastating (nuclear war/meteor impact with few survivors, and a "winter" following, to kill off the old before they can pass on knowledge).
So, to answer your question: In order for a civilization to regress technologically several hundred, or a thousand years, it would require not one, but a series of catastrophic events, usually creating a distrust of technology. It could start with(Year 0):
- Meteor impact (which science failed to predict or stop)
- Nuclear war ("Science killed everyone")
- Robotic uprising.(EMP took them all out but destroyed all of our other tech too)
- Grey goo scenario (See above)
Following this(Year 1-5), people not responsible, or involved in the fall, would start to get angry and ask "why did this happen to us?". Tech state regressed to Industrial age, though space age materials are still in use. No fossil fuels are being produced for vehicles, and so, is running out. Central government is failing to maintain civil order and provide food, water, sanitation and medical supplies. Enter the demagogue.
A charismatic, and manipulative cult leader, gathers the support of the angry, and dispossessed, moving across the continent, looking for more followers and taking what resources they can find. Seeing the growing anti-technology sentiment as something exploitable, the cult leader delivers a series of rousing sermons, demanding that all technologists are sinners and "enemies of God", and should be punished. (Year 5-20). The cult spreads due to a common distaste for technology, and intimidation. Book burning is common, and technology is actively sought out and destroyed.
With a rise in anti-intellectualism, and the persecution of Intellectuals as a group, they are forced underground, and do not pass on their knowledge which is now seen as "witchcraft". Within 50 years, almost all engineering knowledge is lost. Many people do not teach basic numeracy or literacy, which is now the province of those favored of the Church.
History is not taught, expect from the perspective of the church. Centuries of written history are lost within 10 generations (year 50-250), in favor of the teaching of basic skills of survival, such as agriculture, smithery, or fighting. We have now regressed to the middle ages. Places with technology that cannot be easily destroyed are treated as unholy, and avoided by people in general. Like all forbidden things, technology becomes a valued commodity. Modern weapons are still incredibly dangerous, but rarely functional, as the fear of technology ebbs, so does the churches power. People start to actively excavate technology for their own benefit. The age of warlords has begun. (Year 251 - 500)
Warlords conquer with automatic weapons instead of Bows, and rule with terror, using the few remaining grenades. Empires rise and fall within a generation or two. Slavery the vanquished is widespread, as forced manual labor is the only way to feed armies. Ammunition is now so scarce as to be virtually non-existent. As guns and explosives are no longer functional, war regresses to swords, bows and armor. By this time, people have forgotten all modern technology, and the purpose of most industrial technology. Within 500 years, over 1000 years of knowledge, history, and technology is lost. Most cities have been reclaimed by nature, and are now dangerous poorly understood ruins, prone to collapse, and inhabited by wild animals.
Pick any point in this to base your story.