Some factors that contributed to the weakening and decline of medieval europe include:
1) Black Plague - The Black Plague showed that diseases affected everybody, not just common people, and that there is no real 'seperation of class' in the real world.
2) Hundred Year War - This war brought about a large change inside the social structures, as well as how countries ran.
3) Famine - Because of the shortage of workers produced by the Black Plague, people were not able to grow as much food, which led to famine and growing civil unrest underneath the medieval structuring.
hope this helps