Can't reactivate object after I set it to inactivate

the problem is here

  public class test : MonoBehaviour {
 public GameObject bar;
 void Update () { 

     if (Input.GetKeyDown ("k")) {
         if (bar.active == false) {
             bar.SetActive(true); 
         } 
         if (bar.active == true) { 
             bar.SetActive(false); 
         }
     } 
 }
 }
 }

you used 2 ifs ,so for the first time you can deactive the bar,then if you press K again ,in this line

   if (bar.active == false) {
             bar.SetActive(true); 
         } 

you active the bar but at the next line

   if (bar.active == true) { 
             bar.SetActive(false); 
         }

you are deactiving the bar !!!!

so you should change it to this

  public class test : MonoBehaviour {
 public GameObject bar;
 void Update () { 

     if (Input.GetKeyDown ("k")) {
         if (bar.active == false) {
             bar.SetActive(true); 
         } 
         else if (bar.active == true) { 
             bar.SetActive(false); 
         }
     } 
 }
  }

also as that dude said ,you should not attach this srcipt to the bar .because if you press k it will deactive the bar and you cannot use it again.