Error: Ajax Calander Controls stopped working in ASP.Net

VS2010

Sir, 

In my application I have two textboxes which are targeted by two Ajax Calander Controls.  Everything was working fine. All surprisingly the calander control stopped working as it is no more appearing on click inside a textbox control . Though it was working fine .

Pl. guide 

<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>                               
<tr>
    <td class="style7">Period
    </td>
    <td class="style3">
    &nbsp;<asp:TextBox ID="st_date" runat="server" Width="130px"></asp:TextBox>
          <asp:CalendarExtender ID="st_date_CalendarExtender" runat="server" 
            Enabled="True" TargetControlID="st_date">
          </asp:CalendarExtender>
          &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;To
          <asp:TextBox ID="end_date" runat="server" Height="22px"></asp:TextBox>
          <asp:CalendarExtender ID="end_date_CalendarExtender" runat="server" 
             Enabled="True" TargetControlID="end_date">
          </asp:CalendarExtender>