unable to get txtbox text after postback(using jquery dialog)

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jQuery

ASP.NET4

Hi,

I am using jQuery dialog and click send email the email is sent, but I am unable to get the text of TextBox after post back.
Following is the code:

 protected void Page_Load(object sender, EventArgs e)
    {

        if (IsPostBack ==true && Page.Request["__EVENTTARGET"] == "SendReply")
        {
            
            SendReply();
 
        }
.....
}
private void SendReply()
    {
        //ScriptManager.RegisterStartupScript(this.Page, this.GetType(), "KeyToIdentifythisScript", "#dialog_link().click();", true);
        string username = Membership.GetUser ().ToString ();
        if (ViewState["username"]!=null )
        {
body = body.Replace("<%Body%>", txtreply.Text.ToString());// here im getting the text as null 
.....
}

while on page:

$('#dialog').dialog({
            autoOpen: false,
            width: 600,
            buttons: {
                "Send Mail": function () {
                    __doPostBack('SendReply', '')
                   // $(this).dialog("close");
                },
                "Cancel": function () {
                    $(this).dialog("close");
                }
            }

.....
 <table class="ui-accordion">
                                       <tr>
                                           <td>
                                              <p><a href="#" id="dialog_link" class="ui-state-default ui-corner-all"><span class="ui-icon ui-icon-newwin"></span>Contact Information</a></p>
        <!-- ui-dialog -->

        <div id="dialog" title="Contact Information">
            <table>
            <tr>
            <td>Phone Number :</td>
            <td><asp:Label ID="lblContact" runat="server" /></td>
            </tr>
            <tr>
            <td>Reply By Email :</td>
            <td>

                <asp:TextBox ID="txtReply" TextMode="MultiLine"  runat="server"
                    ></asp:TextBox>

            </td>
            </tr>
            </table>

Need help
Thanks