In either a sudden collapse, or gradual decay, much will be lost. Let me remind you that when the Roman civilization decayed, technologies as simple as the making of cement were lost.
Cement.
Not exactly what we'd consider "high tech." It demonstrates just how fragile our scientific advancements are. They can be wiped out by a few generations of relative illiteracy for the great mass of survivors. In three generations, electric lights are a distant legend and those ubiquitous round copper disks find their most frequent use as quick, easily made arrowheads.
Yeah they were knocked back a couple hundred years you knock us back a to 1800 and we would still be able to make electricity Ben franklin was playing with it a decent part of his life. Beyond that the average person in the roman era was illiterate and there has very little written down as apposed to today where every town has at least one public library, the elementary and middle school libraries have a set of one set of encyclopedias each at least, then there is you high schools with chemistry, biology, physics labs and a often a auto shop each with all of the information and much of the equipment need to to bootstrap your way into the early 1900s. Then there are the community colleges which would bring you up to say the 1950 level of tech. Anywhere with a state college or descent sized privet college could probably push you back up to the 1970s if not mid 80s. We despite all of our educations systems failing have at least enough literate people and redundant copies of most enough knowledge to boot strap our tech fairly quickly. Hell anyone with a couple of TB hdd and a few solar cells could mirror more then info information to preserve at elast our access to knowledge.