How to open a modal popup window on Page Load using JavaScript

For a stylish popup in .net use code..... 

   <asp:Panel runat="server" ID="pnlPopUp" style="display:none;">
<div style="position: absolute; width: 100%; background: #333; height: 820px; top: 0;
            left: 0; opacity: 0.6; filter: alpha(opacity=60);"></div>
        <div style="position: absolute; box-shadow: 0 0 3px 3px #333; top: 30%; background: #fff;
            left: 25%; z-index: 1000; min-width:450px;">
   <!--   Add your contents here -->
</div>
</div>
</panel>

at the page load add code as

 pnlPopUp.Style.Add("display", "block"); 

 or  if u r using div instead of panel call script at pageload as

Page.RegisterStartupScript("s", "<script>functionName()</script>");

and script is ..

function functionName()
{
var a=  document.getElementById("pnlPopUp");

a.style.display="block";
}

May this help you.....