How do you display a text box on the GUI after a trigger

So I’m just beginning and I’m writing a simple script so that when the player collides with an enemy it gets destroyed and i want to display a ‘You Lost!’/Game over text. The collision happens, player gets destroyed but text doesnt show, why? or else teach me a better way of doing the same thing. Also showText is defined as a private boolean variable.

    void OnCollisionEnter2D(Collision2D coll)
    {
        if (coll.gameObject.tag == "enemy")
        {
            Destroy(gameObject);
           bool showText = true;
        }
    }
    void OnGUI(bool showText)
    {
        if (showText == true)
        GUI.Box(new Rect(0, 0, Screen.width, Screen.height), "You Lost");
    }

use the new UI system : using UnityEngine.UI; :

public Text GameOver;
void Start(){
GameOver.gameObject.SetActive (false);
}
     void OnCollisionEnter2D(Collision2D coll)
     {
         if (coll.gameObject.tag == "enemy")
         {
             Destroy(gameObject);
             GameOver.gameObject.SetActive (true);
         }
     }