How do you activate a game object ?

I have a question, how can i activate a gameobject ? I can easly deactivate it with this code:

void Start ()
{
gameObject.SetActive(false);
}

But i can’t activate an object if it is deactivated because the script then won’t work so i can’t just write: gameObject.SetActive(true);

Can someone please help me ?

Well, If you have this script on the GameObject that you want to activate/deactivate, this won`t be possible.
I suggest you to put this script in another game object and declare it with tag or public gameobject.

using UnityEngine;
using System.Collections;
     
     public class ActivateObject: MonoBehaviour {
         public GameObject NameOfGameObjectThatYouWantToActivate;
         void Start() {
              NameOfGameObjectThatYouWantToActivate.SetActive(true);
              NameOfGameObjectThatYouWantToActivate.SetActive(false);
         }
     }

Or with tag:

using UnityEngine;
using System.Collections;
     
     public class ActivateObject: MonoBehaviour {
         private GameObject NameOfGameObjectThatYouWantToActivate;
         void Start() {
              NameOfGameObjectThatYouWantToActivate = GameObject.FindGameObjectsWithTag("TagYouPutInYourGameObject");
              NameOfGameObjectThatYouWantToActivate.SetActive(true);
              NameOfGameObjectThatYouWantToActivate.SetActive(false);
         }
     }

I hope helped you!