Why dont work my code behind that call an alert?

Hy,

I have read many threadsandtried in every waybutI can not findthe solutionto invoke a alertfrom my code.
At some pointI have a function

public void Update ()
     {
         if (isSaved)
         {
         }
         else
         {
                Response.Write ("<script> alert ('Hy, im here!') </ Script>");
               ScriptManager.RegisterStartupScript (this, this.GetType (), "script", "alert ('Do you see me?')", True);
         }
}

despiteentering the elsethealert is never generated
I usevisual studio 2008windowsnet framework 3.5and8.1x64

Thanks