Due to (relative to Roman times) high wages and the proliferation of iron throughout the intervening millennia, technology ramped up again in The High Middle Ages which saw the introduction of cranks and wheelbarrows, European copies and improvements upon the mechanical clock, Arabian-style windmills, and the re-introduction of the old three-crop rotation system; the medieval world changed a fair bit in its time, not least because the population of Europe nearly returned to the level it had been in Roman times until the decades before the Black Death, which were marked by population stagnationnote . Even the so-called "Dark Ages" were just a western European phenomenon- the Eastern Roman Empire, the Arab world, the Indian subcontinent and the Chinese realms were - though constantly in flux, with major fluctuations in population and general prosperity levels due to the wars and chaos accompanying the rise and fall of several different empires - largely as militarily and culturally sophisticated as ever, their empires having done much better to resist the encroachment of the peoples of the Steppes (Mongol-led Empire of the Yuan aside). The idea of a medieval decline is a trope in itself, which has been around since the Renaissance. Many forms of media focus on historical inventions because it seems logical that scientific advancement and inventions result in economic growth. As far as they are concerned, this is the story of human development - Science resulting in Modernity. Actually, the opposite is true; necessity dictates the adoption, or not, of technologies with practical applications. This fact of history is why engineers, historians, and economists alike laugh at the idea that Archimedes alone could have started the Industrial Revolution two millennia early. The Roman Empire, the Empire of the Song, and the Mughal Empire were all pretty damn big and culturally sophisticated, but their populations didn't craft precision optics or found joint-stock companies or use square-rigged sails because there was no reason for their peoples to use those things. In other words, necessity is the true mother of invention. Homo erectus adopted the technology of fire not because Prometheus told him to invent technological wonders to rival God; it's simply because fire defended humans against nocturnal predators, and also made food easier to digest, parasite-free and safer to eat, thus more families and less roundworms. Desert civilizations such as Mesopotamia and Egypt invented farming because hunting-gathering in a desert will just kill you, square-rigged sails became popular because of colonial incentives, while Renaissance Europe and the Soviet Union had to be forced into a Lensman Arms Race as their incentive for invention (if there was no Colonial Era/Cold War to force a Lensman Arms Race, the sheer amount of anti-intellectualism in the Inquisition-era/Communist Countries would have forced them back into a famine-ridden dark ages). Then again, for us modern humans, inventing nature-controlling technology for the sheer pleasure of it may count as a necessity too. Scientific progress, moreover, was 'very' slow-paced from c.1400 to about c.1800 (even at which time the use of the so-called 'Scientific Method' was still far from universally accepted) and had no practical applications beyond navigation, time-keeping, and of course war - most European people, even in the more advanced parts of Europe like the Dutch Republic, were dirt poor and died young just like their compatriots in Colonial America, Bengal, the Sudan, Java, and Guangdong. The nature of most changes that occurred in that period concerned weaponry, the deployment of weaponry, and institutions for making and using and acquiring the money to make and use weapons - it was only with the advent of the industrial revolution that people started becoming better-fed, living a bit longer and dying less. Indeed, it was only in 1865 that London completed its public water- and sewer-works; the first of its kind in the world, it was decried by the scientific community as a shameful example of governmental spinelessness in caving in to the demands of the so-called 'sanitarian' pseudo-scientific movement, which was making wild and unsubstantiated claims that the use of soap and general cleanliness would reduce the incidence of disease. Continual technological progress has never been inevitable in human history, even though modern capitalism makes it feel inevitable in developed countries today. Different cultures invent new technologies at different rates, depending on the available resources, their needs for survival/pleasure/social status, cultural inclinations, historical events, and so on. Many peoples never invented writing, or ceramics, or wheeled vehicles, or in some cases not even agriculture. Cultural anthropologists and archaeologists don't have one universal answer for why this is — reasons can include lack of necessary resources (minerals, animals, plants, population, climate, energy, etc.), cultural aversion to a particular technology, or just plain not needing to invent something new. In some cases, once widespread technologies have been abandoned when the need or resources disappeared. While all human cultures continually adapt the technology they already have to environmental and social changes — and thus true stasis never happens — people don't invent entirely new technologies without some economic, social, military, or other incentive driving them to want to. Anthropologists are just as interested in figuring out why various technologies were invented at all, as in why some cultures didn't invent them.
Go back to Medieval Stasis and leave your computer here, will you?