what is wrong with the below simple code?

in a button click I am trying to display a modal popup window
so in the ,cs side button1_click
 Modalpopupextender2.show();
But it is not displaying any thing on the button click. What is missing?
 
<style type="text/css">
     .modalBackground
    {
              background-color:Grey;
              /*Olive; */
              filter:alpha(opacity=70);
              opacity:0.7;
    }
   
     .modalPopup
    {
              background-color:#ffff ...