The fall of Rome is kind of a special case; aside from the survival of the Eastern Empire mentioned above, there was also the Catholic Church around to preserve at least a bit of ancient knowledge, to keep some level of cultural unity across the former Empire, and to facilitate the reintroduction and transmission of ancient texts which had vanished in the West but survived in the East and the Islamic world. Butterfly away Christianity and the fall would be a lot more traumatic, IMHO.
As for civilisations collapsing, in some ways the more sophisticated they are, the more vulnerable they are to collapse. For example, the collapse of the Roman Empire was bad enough, but even at the Empire's height most people (c. 90%, if memory serves) were working on the land, so they'd have at least known how to grow their own food. Also, they'd have generally able to support themselves on their own crops even when the long-distance trade routes were disrupted. Now compare that to the modern West, where around 2% of people are engaged in agriculture and we import a significant amount of our food. Any collapse of Western civilisation comparable to the fall of Rome would almost certainly result in mass starvation.
But, if you want more of a gradual decline rather than a catastrophic collapse, maybe a plausible candidate would be for fossil fuels to gradually run out (/become too expensive for ordinary domestic use) without us being able to find a renewable alternative energy supply instead. If the decline takes a sufficiently long period of time (probably a few generations at least), people would be able to adapt without too much havoc being wreaked on society, but we'd still have to, essentially, go back to pre-electricity days for most things. Also, long-distance transport would become more difficult, so there'd be less globalisation and long-distance trade going on. Would that be suitable enough for the parameters of the OP?
A post-1900 collapse via peak oil sounds fun. But I'm mostly setting my sights on pre-1900 societies, before coal and oil was in high demand. Modern and early modern civilization would seem to have lots of redundancies in place. If one society collapses, another just takes advantage of its weakness.
I suppose in some regard modern regions have 'collapsed' in a sense, that is to say, they have seen much better days. Areas such as the Balkans and the Middle East IOTL, which used to be quite powerful and influential places but in recent decades become rather war-torn and shadows of their former selves, with a few exceptions. I suppose one could do that with early modern civilization.
Collapse, especially in this regard, can definitely get some book-burning and general forgetting of technologies and secrets but I'm still trying to confirm if civilization can simply regress, probably under the influence of stagnation or something. Some sort of trend where older ways appear to be more desirable or the current ways seem unfeasible, nations look to cut the fat off of what they see as frivolities and simplify some things...of course, this would probably have to mean little threat of competition, but I'm not sure. Books seem to be the primary hurdle in preventing a loss of knowledge and, well, 'negative' technological progress.
No, it isn't. Middle Eastern peoples abandoned the wheel in favor of the camel, for example, since camels were actually more efficient than wheeled transport for what they needed. That said, technological advances, when they are useful in making life easier, do get adopted.
Oh yeah, that's right! But they never really forgot the wheel, did they? Just stopped using it because the ancient roads became impractical.