The target variable in your LookAt monobehaviour needs to somehow be updated to the new gameobject. One of the ways to achieve this is to make the code that does the respawning, also inform the lookat script about the "new thing to look at".

The variable will be empty as it is a reference to a gameobject that's not part of the prefab. A simple solution is to set the variable through code, this would give something like this:

var target : Transform;

function Start() {
    target = GameObject.FindWithTag("Player").transform;
}

Of course, this assumes you gave the player Tag Player and that you're interested in the transform of the player. If you want to store the GameObject of the player, replace Transform with GameObject and remove the .transform.