Ajax calendar extender pop up calendar not displaying on click of targetcontrolid

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ASP.NET

Ajax

I am using ajax calendar extender. I have a textbox and calendar image next to it. When user clicks on image button, the calendar is to popout and on selection of date, the date is to be displayed on textbox. The client side code is

<asp:ScriptManager ID="sm" runat="server">

<label id="lbldate" runat="server" class="editor-label">Date </label>
       <asp:TextBox ID="txtStartDate" runat="server" Width="155px" style="background-color:whitesmoke;" AutoCompleteType="None" ></asp:TextBox>
        <asp:Image ID="imgCalendarStartDate" runat="server" ImageUrl="~/Styles/Images/Calander.png" Height="16px" Width="16px" />
       <ajax:CalendarExtender ID="ajaxCalendarExtender" runat="server" BehaviorID="ajaxCalendarExtender" TargetControlID="txtStartDate" PopupButtonID="imgCalendarStartDate"></ajax:CalendarExtender>

Here, as i click the calendar image button, the calendar does not pop up.. is there anything i have missed.
Thanks in advance...