Hi, this is a very common mistake that people got "stupider" after the fall of Rome or any significant amount of knowledge 'disappeared' after the fall. We have a pretty extensive FAQ which gathers some of the better answers to them. The first one I believe is the most comprehensive but I'll post another of my favorite ones:
The issue is not that people "got dumber" or "fell behind" but rather Europe went through a massive fragmentation process and it created the illusion of 'dumbing down' not because people were dumber but because people just fundamentally held less land, had less people, and less money and trade connections.
The Romans could build these enormous stone monoliths in central Italy even though Italy isn't exactly a stoneworks producing region because they could sail in thousands upon thousands of tonnes of stone from across a safe Mediterranean (because they owned all of it) from territories they owned across it for essentially "free". However how can we expect a continuous series of successor states all fighting each other for the territory now being divvied up and trying to consolidate themselves, without the trade connections and sheer size of empire, be expected to do the same scale of works?
Why would a small Count or Duke need an aqueduct? Or an extensive series of roads? How can someone who is trying to just keep their small plot of land together justify building a giant Roman-like temple? With no strong centralized power along with a continuous series of invading powers Europe just became a fragmented place that frankly did not have the time nor inclination or funds/justification to do the splendorous things Roman's did.
We see that right when things started to stabilize these things start coming back but bigger and better. We see the rise of the University, modern medicine, absolutely beautiful and intricate Gothic Cathedrals. The Medieval Era, especially post-1000, is home to such an extensive list of engineering feats and technological inventions it would be redundant to go through them all. Independently developed gunpowder, glasses, clocks, modern papermaking techniques, Arab numerals, the compass, astrolabs to calculate latitude, cross staffs, artesian wells which didn't need to be pumped, advancements in mill and windmills which removed the need for manual labor, horseshoes, and that's not even going into the laundry list of military inventions the Medieval Era was home too.
So they never really "fell behind" they just had more priorities than competing with and continuing on the masterful works Rome was capable of doing out of the luxury of it holding just about the entire Mediterranean (and then some) with a strong, centralized bureaucracy in the middle of it all. However, if we just include ourselves to the Eastern Roman Empire there was still quite the list throughout this period as well.
