at postback page textbox value is clear

<%@ Page Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true" CodeFile="Default2.aspx.cs" Inherits="Default2" Title="Untitled Page" %>

<%@ Register assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" tagprefix="cc1" %>

<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
    <asp:TextBox ID="TextBox1" runat="server" Enabled="False" ReadOnly="True"></asp:TextBox>
    <cc1:CalendarExtender ID="TextBox1_CalendarExtender" runat="server"
        Enabled="True" PopupButtonID="Button2" TargetControlID="TextBox1">
    </cc1:CalendarExtender>
    <asp:Button ID="Button2" runat="server" Height="25px" Text="+"
        Width="39px" />
    <br />
    <asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="add" />
</asp:Content>