First, one must describe feudalism: this was a practice where nobles would give the king labor and military loyalty in return for land and favor. This was a very stratified society with the king at the top, with lords, followed by knights, and then peasants and laborers. It did not end uniformly in Europe, and Russia continued to use a form of feudalism until the 1800s with serfdom.
Now that we have discussed the practice, we can look at what ended it. The Black Death killed over a third of Europe and killed even more in localized places like Marseilles and Venice. This destroyed the local labor force and made the peasants demand higher prices--there was now more land than there was people to work it. The Peasant Revolt also demonstrated how fragile the system was. The economies moved to a cash base instead of a land base--this meant that monarchs improved their taxation systems and economies moved to market economies. Armies grew larger and gunpowder technology meant that armies did not need extensive training--this was the time that military science became a subject for generals to study. The Crusades also created more trade opportunities, thus opening up other revenue streams other than land for monarchs.