Please refer this code
HTML
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js" type="text/javascript"></script>
<script type="text/javascript">
$(function () {
if ($('[id*=ddlShowHidePanel]').val() == 'Y') {
$('[id*=Panel2]').hide();
$('[id*=Panel1]').show();
}
else {
$('[id*=Panel2]').show();
$('[id*=Panel1]').hide();
}
$('[id*=ddlShowHidePanel]').change(function () {
if ($(this).val() == 'Y') {
$('[id*=Panel2]').hide();
$('[id*=Panel1]').show();
}
else {
$('[id*=Panel2]').show();
$('[id*=Panel1]').hide();
}
});
});
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:DropDownList ID="ddlShowHidePanel" runat="server">
<asp:ListItem Text="Y" Value="Y" />
<asp:ListItem Text="N" Value="N" />
</asp:DropDownList>
<asp:Panel ID="Panel1" runat="server">
<asp:TextBox ID="txtPanel1" runat="server" Text="Panel 1" />
</asp:Panel>
<asp:Panel ID="Panel2" runat="server">
<asp:TextBox ID="txtPanel2" runat="server" Text="Panel 2" />
</asp:Panel>
<asp:Button Text="Save" runat="server" OnClick="Save" />
</div>
</form>
</body>
</html>
C#
protected void Page_Load(object sender, EventArgs e)
{
if (!this.IsPostBack)
{
this.ddlShowHidePanel.Items.FindByValue("Y").Selected = true;
}
}
protected void Save(object sender, EventArgs e)
{
if (this.ddlShowHidePanel.SelectedItem.Value == "Y")
{
//Save txtPanel1 Text
}
else
{
//Save txtPanel2 Text
}
}