<asp:CollapsiblePanelExtender ID="cpeTest" runat="server" TargetControlID="Panel1"
                                            CollapsedSize="0"
                                            Collapsed="True"
                                            ExpandControlID="lbutton1"
                                            CollapseControlID="lbutton1">

 cpeTest.ClientState = "true";
 cpeTest.Collapsed = true;