In the Middle Ages, there was virtually no literacy in Europe except within the Church societies. Really, this was not much of a significant change from the ancient world since most people in the ancient world had little use for written materials. Anything of value for your occupation was transmitted orally from master to apprentice. There was simply no need for it outside of the Church.
Many of the European languages such as Frankish, Celtic and the Scandinavian languages had no written form anyway and were more conglomerations of various local dialects than they were broadly spoken languages.
On top of that, most of the Mediterranean world spoke either Latin or Greek in the Roman period, and as Roman power retreated from Europe, the existing peoples – mostly Celtics or Germans – adopted Latin to their purposes. As a result, they developed a number of Romance languages – languages based on Latin.
These languages which were referred to as vernacular or “household”, took on characteristics of their own during the medieval period but were not used as written languages until after the year 1000 as these new national identities began to take hold and form their own corpora of literature. During the next few hundred years, reading and writing in one’s own language became more and more part of your identity as part of a nation, and that in turn standardized the vernaculars into more formal languages.
The fall of Constantinople (1453) and invention of the moveable type printing press (1436) sounded the end of the medieval ages and the illiteracy that accompanied it.