How did the ideas of the philosophers of ancient Greece and Rome get lost for so many yers?

With the collapse of the Roman Empire resulting from the takeover by the incoming peoples from Eurasia, the remnants of learning were kept in the monasteries, but at a very low level. Much of the knowledge was preserved in the more enlightened of the Islamic empires, which filtered back into Europe as it passed its dark ages and emerged from the Middle Ages into the Enlightenment.