Due to their immense size, the classical empires had continuing problems of internal instability and external invasions. By 500CE, all these major empires had fallen or weakened. Persia was still limping along under the Sasinids, while the Hellenistic Empires had come under increasing control of Rome- which was now only the Eastern half (Byzantine Empire) as the west had fallen to Germanic invaders. The Han in China too had become weak from social conflict between wealthy and poor over land distribution, as well as continuing battles along the northern border with the nomadic Xiongnu. The Post-Classical era ushered in smaller kingdoms in Europe and India, but also greater interconnectedness through trade now centered in the Indian Ocean with the increased dominance of Arabian merchants and the Chinese dynasties of the Sui, Tang, and Song. Now, instead of forced unity in large empires, trade brought these splintered regions together and increasingly moved products, people, ideas, and disease throughout Eurasia and now Africa.
Bonus Material!!!! This material will help to enhance your understanding of the material :)
Reading: Marco Polo and Ibn Battuta: the Merchant and the Pilgrim