I don't know based on the rulebook.

The provided rulebook context does not specify what happens to a player's units if their last Haven is removed from the board, nor does it explain whether or how a player can re-enter the game by building a new Haven after losing all Havens. The rules describe how to build Havens (page 23), how Havens interact with units and defenses (pages 17, 23), and how production works with Havens (page 29), but do not address the situation of a player with no Havens remaining.

To answer this question, the rulebook would need to include a section on player elimination, recovery, or special rules for players without Havens.

Source: Core Rulebook (pp. 11, 17, 21, 23, 29)