It may refer to the second-last king of Judah who was taken in exile to Babylon along with the administrative elite of the land in 597 B.C. This history is recorded in 2 Kings 24:14–15.1 While he was released from prison around 562 B.C., he never returned to the land.
It may also refer to the last king, Zedekiah. Zedekiah tried to escape Jerusalem with his sons but was captured. His sons were killed before him and then he was blinded. He was dragged in chains to Babylon, never to be heard of again. This history is recorded in 2 Kings 24:1–20 and Jeremiah 52:1–34.2
Another possibility is that it may refer to both kings.
9 Her gates have sunk into the ground; he has ruined and broken her bars; her king and princes are among the nations; the law is no more, and her prophets find no vision from the LORD.