button event not firing in ajax modelpopupextender

<ajaxToolkit:ModalPopupExtender ID="ModalPopupExtenderLogin" runat="server" BehaviorID="ModalPopupBehavior"
          DropShadow="true" OkControlID="btnok" TargetControlID="lblmsg"  PopupControlID="LoginPanel" />
  <asp:Panel ID="LoginPanel" runat="server" Style="clear: both;" Height="227px" Width="372px">
      <asp:ScriptManager ID="scriptmanager2" runat="server">
      </asp:ScriptManager>

      <table border="0" cellpadding="3" cellspacing="1" width="374px" style="height: 180px;">
          <tr>
              <td style="width: 200px" align="center">
                  <span><b>Old Password:</b></span>
              </td>
              <td style="width: 175px" align="left">
                  <asp:TextBox ID="txtoldpassword" runat="server" TextMode="Password" TabIndex="1"
                      MaxLength="20"  ValidationGroup="ChangePassword" ></asp:TextBox>
              </td>
          </tr>
          <tr>
              <td style="width: 200px" align="center">
                  <span><b>New Password:</b></span>
              </td>
              <td style="width: 175px" align="left">
                  <asp:TextBox ID="txtNewPassword" runat="server" TabIndex="2" TextMode="Password" ValidationGroup="ChangePassword" ></asp:TextBox>
              </td>
          </tr>
          <tr>
              <td style="width: 200px" align="center">
                  <span><b>Confirm New Password:</b></span>
              </td>
              <td style="width: 175px" align="left">
                  <asp:TextBox ID="txtConfirmNewPassword" runat="server" TabIndex="3" TextMode="Password"  ValidationGroup="ChangePassword" ></asp:TextBox>
              </td>
          </tr>
          <tr>
              <td colspan="2" align="center">
                  <asp:Button ID="btnok" runat="server" CausesValidation="false"  Text="OK" OnClick="btnok_Click" ValidationGroup="ChangePassword" />&nbsp;
                  <asp:Button ID="btncancel" runat="server" Text="Cancel" />
              </td>
          </tr>
      </table>
  </asp:Panel>