CKEditor Not visible after post back and jquery validation not working

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C#

ASP.NET

I am using CKEditor 3.6.4 version in my asp.net page. when page loads for first time it will appear. But if am select something in dropdownlist then page will be postback then CKEditor disappears. i am using jquery validation for my page. validation also not working for CKEditor..
How to solve this issues..?

<script type="text/javascript" src="ckeditor/ckeditor.js"></script>
<script type="text/javascript">
window.onload = function () {
var config = { toolbar:
[
['Bold', 'Italic', 'Underline', '-', 'NumberedList', 'BulletedList', '-', 'Link', 'Unlink']]};
CKEDITOR.replace('DetailsTxtbx', config);
}</script>

<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<contenttemplate>

<asp:TextBox ID="DetailsTxtbx" runat="server" CssClass="validate[required]" TextMode="MultiLine">

<asp:Button ID="Button1" runat="server" CssClass="button"
onclick="Button1_Click" Text="Submit" />