How do I make the objects appear again after they disappeared

Thanks for the answer! I posted it yesterday and as its my first project, I changed it to make it easy. Now what I want is, when one close to get out of the display, make it returns backwards. I tried to make it but since I can't use "while" loop because of crashes, when object come to X or Y end point, it changes the turn but it also turns back the first turn again.

Sample:

 void update

 float a = _myRigidbody.transform.position.x;
 float b = _myRigidbody.transform.position.y;
 _myRigidbody.velocity = new Vector2 (15, _myRigidbody.velocity.y);
  
 if (a > 20) {
 GameObject.Find("Coin").transform.position = new Vector2(a-1,b);
 myRigidbody.velocity = new Vector2(15, _myRigidbody.velocity.y);
 }