The fourteenth century was one of the most disastrous. There were three major causes for this.
First, there was climate change. The temperature began to drop slightly. This might sound like a minor thing, but when we look at how temperature affects agriculture, then we will realize the gravity of the situation. For example, in France, crops began to fail in 1315. This caused famine and there were reports of cannibalism.
Second, the bubonic plague struck Europe. Historians call this the Black Death for good reason, as a significant portion of the population contracted the disease and died. Low estimates state that one in three died of the disease. More likely, half of the population died. From this perspective, the bubonic plague was tantamount to a nuclear bomb.
Third, there was also social upheaval. The Hundred Years' War lasted from 1337-1450. This war caused many casualties in an already weakened Europe.
There were two major causes of the population decline in Europe during this century.
First, there was the "little ice age." This was a time of slightly colder temperatures that made growing seasons shorter and brought a lot more rain to much of Europe. Both of these reduced crop production and caused famines across Europe. Some historians believe that these famines caused a decline of around 10% in the population.
Then, in 1347, the Black Death hit Europe. From then until the end of 1350, it spread across the continent. The Black Death is believed to have killed somewhere between 25% and 50% of the population of Europe in just three years.