You might not need to worry about how big the event is but rather show how a series of smaller events triggers a decent into ignorance. The Movie 'Naausica of the Valley of the Wind' by <PERSON> has a cataclysmic war named "The Seven Days of Fire" but some tech survived but society declined back to an interesting mix of bronze-age with sprinkles of tech. It might serve as inspiration for you.
There are other answers that suggest the same thing and I'll give my two cents as to why it is a better approach. Disasters of large enough scale to almost destroy humanity are both easy to imagine, nuclear war or a meteor strike, that they have a fairly large body of literature exploring the concept and also common enough to become a trap for lazy authors. 55
Those large-scale catastrophes are too easy to use as a deux ex machina to drive the plot or explain the history.
How much more interesting would it be to show how small events chain together to doom civilization, such as the last copy of an important manual being tossed on a fire by anti-tech zealots, or a lab that can produce antibiotics laying idle because no one knows how to fix a generator, or relatedly, the fuel needed to run the generator being hoarded by a survivalist who dooms himself and his family because they need the drugs the fuel would have allowed to be produced.
I know other answers have suggested this shows how fragile civilization is and I want to clarify that this is not what I mean by this answer - the difference being that it's not civilization that is fragile, it is the human psyche that can introduce instability when given the right circumstances. Society drives people to prefer safety of those in the 'in' group over much else so if you get the wrong person in charge they can wreck havok, the question remains then how large the effect is and how different tribes interact when their leaders are at odds or irrational or misguided. Now do that on a larger scale and over a few generations and it's easy to end up with a globe full of ancient tech that no one knows how to use.
Get a small scale disaster to start the ball rolling and show how the short-term needs of a group start dictating their response at the expense of their long-term needs. Have you studied the fall of the Roman Empire? It didn't fall overnight but it had a similar effect as your question. You might consider exploring the space with a short story that follows 'the last engineer in Rome', as in an educated person who no longer has the resources or support to keep things in repair such that the world succumbs to ignorance when he dies. 159