What threats did europeans face after the fall of the roman empire

When the western Roman empire collapsed in 476 AD, the unifying force for most of Europe was gone. Along with the loss of the government, much of the engineering abilities and "modern amenities" were also lost. The "barbarians" of Europe did not, for the most part, have the abilities or knowledge to build roads, aqueducts or the other marvels of the Roman Empire, which is why the period immediately after Rome's collapse is often referred to as the "Dark Ages".

The only unifying force of any sort left would be Constantine's Roman Catholic church, which did manage to preserve some of the knowledge and vestiges of the empire. As the only thing left really, it became the main power in the European world throughout the Middle Ages, functioning almost as an imperial government until Luther set off the Reformation in 1546.