What do you mean, exactly? Nothing really "happened" to them, they stayed where they were and continued the development of their society until they eventually became the Greeks we're familiar with in the Classical period.

Their palatial societies seem to have crumbled during what we call the Late Bronze Age Collapse, and it's possible there were migrations into the eastern Aegean, but for the most part the same people from the Broze Age turned into the societies of the Iron Age.

This collapse is sometimes called the "Greek Dark Age," though recent evidence has come to light that shows this age may not have been as dark as we once thought.

I'm reading a great book by Robin Lane Fox called Travelling Heroes that describes through archaeological and textual evidence from across the Mediterranean how Euboean Greeks flourished during what we sometimes call the post-Mycenaean "Dark Age".