Many were never rebuilt.

The cities of Ugarit on the Syrian coast, Megiddo and Hazor in Canaan, and numerous other centers were burned.

Trade in the eastern Mediterranean collapsed.

The palace administrative systems disappeared.

The writing systems used by the Mycenaeans and the Hittites vanished — the Greeks would not write again for several centuries, until they adapted the Phoenician alphabet to their language around 800 BCE.

This was not a gradual decline.