How to activate all inactive objects?

Search for gameobjects and store them in an array. Then pass through that array and activate them.

 public void activateObjects() {
         GameObject[] gameObjects = GameObject.FindGameObjectsWithTag("TAGNAME");
         foreach (GameObject go in gameObjects)
         {
             if (!go.activeInHierarchy)
             {
                 go.SetActive(true);
             }
         }
     }