Given that your lantern archon was a cohort, my guess was that it was a called outsider or one that had traveled to the Material Plane on its own. In that case, it's dead when slain, and by the core rules, you need a wish or miracle to bring it back. However, if you use the 3e Manual of the Planes, there is a spell in that book called revive dead outsider (Clr6) that works just like raise dead for outsiders. You could easily afford that spell, I imagine, and if your DM allows it, you could have it cast to bring back your archon buddy.