Event coudnt fire on modalpop control

//source code

<asp:ScriptManager ID="ScriptManager1" runat="server">



<asp:LinkButton ID="Lnkbtn_chngpass" runat="server"
onclick="Lnkbtn_chngpass_Click">Change Password
<cc1:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="Lnkbtn_chngpass"
CancelControlID="Btncancel" OkControlID="Btnok" PopupControlID="Panel_chngpass" BackgroundCssClass="ModalPopupBG">


<asp:Panel ID="Panel_chngpass" runat="server">

<asp:Label ID="Lbl_currntpass" runat="server" Text="Current Password">
<asp:TextBox ID="Txt_currentpass" runat="server" TextMode="Password">
<br/>
<asp:Label ID="Lbl_newpass" runat="server" Text="New Password">
<asp:TextBox ID="Txt_newpass" runat="server" TextMode="Password">
<br />
<asp:Button ID="Btnok" runat="server" Text="Change Password"
onclick="Btnok_Click" />
<asp:Button ID="Btncancel" runat="server" Text="Cancel"
onclick="Btncancel_Click" />