how to show disabled textbox when dropdowns selcted item is complete?

 <script type="text/javascript"> function changetextbox() 
{     if (document.getElementById("ddl").value === "noy")
 {         document.getElementById("txt").disable='true';     } 
else {         document.getElementById("txt").disable='false';     }
 } 
</script> 

<select id="ddl" name="ddl" onChange="changetextbox();"> 
    <option>Yes</option>     <option>No</option>
     <option>No, but planning in future</option> </select>
  <input type="text" name="txt" id="txt" /> 

if(dropdonws.SelectedItem.Text=="Completed")
{
textBox.ReadOnly=true;
}

if(dropdonws.SelectedItem.Text=="Completed")
{
textBox.Enalbed=false;
}

if(dropdonws.SelectedItem.Text=="Completed")
{
textBox.disabled=false;
}