The forceful intervention of the Egyptian empire in the internal affairs of the Kingdom of Judah reaches a breaking point with the removal of the sitting king and the imposition of heavy economic sanctions on the nation. The king of Egypt brings an abrupt end to the Judean monarch's rule, stripping him of power and placing him in prison.
While the fall of the king is tied to his capital city, the parallel historical account in the Book of Kings reveals that Pharaoh Necho actually imprisoned him in Riblah, located in the land of Hamath, rather than inside Jerusalem itself. Because of this, the primary approach among commentators is that the king was not physically captured in the capital. Instead, the focus is purely on his loss of authority; he was removed from his royal seat and his position as the ruler over Jerusalem [רש״י, רד״ק, רלב״ג].
Following the king's removal, the Egyptian monarch turns his attention to penalizing the entire kingdom. He imposes a severe financial fine and a heavy tax on the country. The people of Judah are forced to comply with this demand because the military power rests entirely in the hands of the Egyptian ruler [רש״י, ביאור שטיינזלץ].
This heavy national penalty is not arbitrary, but rather a direct consequence of the people's own actions. The fine is levied against the nation as a punishment for interfering with the natural order of royal succession. The people had taken it upon themselves to crown Jehoahaz over his older brother, Jehoiakim, who rightfully held the legal claim to the throne [מצודת דוד].