protected void btnExport(object sender, EventArgs e)
{
DataTable resultToExcel = ExportToExcel.ExportDataToExcel(WebConfigurationManager.ConnectionStrings["строка подключения"].ConnectionString);
//ExportToExcel - класс, ExportDataToExcel - метод   ПОЛУЧАЕМ ДАННЫЕ ИЗ БАЗЫ

HtmlTable results = ExportUsersInManager.ConvertToHtml(resultToExcel);//ExportUsersInManager - конвертировать DataTable в HtmlTable
           Response.Clear();
           Response.Cache.SetCacheability(HttpCacheability.NoCache);
           Response.AddHeader("Content-Disposition", "attachment; filename=Тест.xls");
           Response.ContentType = "application/vnd.ms-excel";
           StringWriter stringWrite = new StringWriter();
           HtmlTextWriter htmlWrite = new HtmlTextWriter(stringWrite);
           results.RenderControl(htmlWrite);
           Response.Write(stringWrite.ToString());
           resultToExcel.Dispose();
           results.Dispose();
           Response.End();
}

public static class ExportToExcel
{
public static DataTable ExportDataToExcel(string Connection)
{
DataTable resultsToExcel = new DataTable();
//если надо что то делаешь
using (SqlCommand command = new SqlCommand(store))
{
//добавляешь параметры если нужны
SqlDataAdapter adapter = new SqlDataAdapter(command);
            try
            {
                adapter.Fill(resultsToExcel);
            }
            catch { return null; }
return resultsToExcel;
}
}
}

public static class ExportUsersInManager
{
public static HtmlTable ConvertToHtml(DataTable dataTable)
    {
        var htmlTable = new HtmlTable();
        if (dataTable == null) return htmlTable; //null reference, empty table

        HtmlTableRow htmlRow = new HtmlTableRow();
        htmlTable.Rows.Add(htmlRow);

        foreach (DataColumn column in dataTable.Columns)
        {
            //htmlRow.Cells.Add(new HtmlTableCell() { InnerText = column.ColumnName });
            HtmlTableCell Cell = new HtmlTableCell();
            Cell.InnerText = column.ColumnName;
            Cell.BgColor = "Gray";
            Cell.Align = "Center";
            Cell.Attributes.Add("style", "font-size:14px; color: white;");
            htmlRow.Cells.Add(Cell);
        }
        
        foreach (DataRow row in dataTable.Rows)
        {
            htmlRow = new HtmlTableRow();
            htmlTable.Rows.Add(htmlRow);

            foreach (DataColumn column in dataTable.Columns)
            {
                //htmlRow.Cells.Add(new HtmlTableCell() { InnerText = row[column].ToString() });
                HtmlTableCell Cell = new HtmlTableCell();
                Cell.InnerText = row[column].ToString();
                htmlRow.Cells.Add(Cell);
            }
        }

        return htmlTable;
    }
}