protected void Page_Init(object sender, EventArgs e)
{
    var path = Server.MapPath("~/App_Data/spreadsheet.xlsx");
    var workbook = Workbook.Load(path);
    HiddenField1.Value = workbook.ToJson();
}

<script>    
    function pageLoadHandler() {
        var spreadsheet = $find("<%= RadSpreadsheet1.ClientID %>");
        var hiddenfield = $get("<%= HiddenField1.ClientID %>");
        if (hiddenfield.value) {
            spreadsheet.get_kendoWidget().fromJSON(JSON.parse(hiddenfield.value))
            hiddenfield.value = '';
        }

        // Sys.Application.remove_load(pageLoadHandler);  
    }
    Sys.Application.add_load(pageLoadHandler);
</script>
<telerik:RadSpreadsheet runat="server" ID="RadSpreadsheet1"></telerik:RadSpreadsheet>
<asp:HiddenField runat="server" ID="HiddenField1" />