The hellenic world really had some cool gadgets. Regarding the middle ages, for our german language readers, there's an
excellent article in the german Wikipedia.
Sadly I couldn't find an equivalent in the english one.
It is apparently established by science that the loss of antique knowledge did not take place in the middle ages, but way before it, between 300 and 560 AD.
Antique knowledge was lost while the roman empire still existed!
Conclusion of the article was "most ancient books were definitely lost before 800, most likely even before 500 AD".
Evidence for that is the fact that even the byzantine empire did not have more antique literature than we have today, and Constantinople was never plundered by anyone before 1203.
Also, early medieval catalogues do not list books we do not have.
It seems that many in the middle ages actually tried to preserve what was still there, there's hardly any work known to have been lost in the middle ages.
In contrary, the crusaders brought back many works lost in the west that had been kept by the byzantines, arabs or persians, for example most of the works of Aristotle.
So if anything, antique knowledge increased in the middle ages, not decreased.
However, this was limited to a small percentage of people.
Weird thing is, the sheer volume of literature of the antique age was only surpassed in the 19th century.
Same as people in Europe only enjoyed the same kind of sanitation and standard of living as the average roman in the 19th century.
So why were those books lost in the late roman empire?
Sources are scant, but it seems the rise of christianity brought along a cultural revolution worse than that of Mao.
Not only christianity, but a number of religions competed for the average romans salvation in the late empire, and all of them did not like the old ways.