how to activate a inactive gameobject after starting the game?

I want to show a panel with text when i will click the player. so i deactivate the panel in the scene by unchecking the box in the inspector. But when i get the panel by FindGameObjectWithTag() and i want to set it activate with the function OnMouseDown(). But when i clicking the player to show the panel its shows null references. If i activate the panel initially then the OnMouseDown() working perfectly. But i don’t want that. i want to appear it when i want in a running game. I have tried with this.

pnl = GameObject.FindGameObjectWithTag (“Panel”);
void OnMouseDown(){
if (pnl.activeSelf == false) {
pnl.SetActive (true);
} else
pnl.SetActive (false);
}

use can use :
gameObject.enabled= false;