Hi gkoutsiv,

Check this example. Now please take its reference and correct your code.

Database

I have made use of the following table Customers with the schema as follows.

I have already inserted few records in the table.

You can download the database table SQL by clicking the download link below.

Download SQL file

HTML

<asp:Button Text="Export" OnClick="ExportExcel" runat="server" />

Namespaces

C#

using System.IO;
using System.Data;
using ClosedXML.Excel;
using System.Configuration;
using System.Data.SqlClient;

VB.Net

Imports System.IO
Imports System.Data
Imports ClosedXML.Excel
Imports System.Configuration
Imports System.Data.SqlClient

Code

C#

protected void ExportExcel(object sender, EventArgs e)
{
    if (!Directory.Exists(Server.MapPath("~/Files")))
    {
        Directory.CreateDirectory(Server.MapPath("~/Files"));
    }
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT * FROM Customers"))
        {
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {
                cmd.Connection = con;
                sda.SelectCommand = cmd;
                using (DataTable dt = new DataTable())
                {
                    sda.Fill(dt);
                    using (XLWorkbook wb = new XLWorkbook())
                    {
                        wb.Worksheets.Add(dt, "Customers");
                        wb.SaveAs(Server.MapPath("~/Files/SqlExport.xlsx"));
                    }
                }
            }
        }
    }
}

VB.Net

Protected Sub ExportExcel(sender As Object, e As EventArgs)
    If Not Directory.Exists(Server.MapPath("~/Files")) Then
        Directory.CreateDirectory(Server.MapPath("~/Files"))
    End If
    Dim constr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Using con As New SqlConnection(constr)
        Using cmd As New SqlCommand("SELECT * FROM Customers")
            Using sda As New SqlDataAdapter()
                cmd.Connection = con
                sda.SelectCommand = cmd
                Using dt As New DataTable()
                    sda.Fill(dt)
                    Using wb As New XLWorkbook()
                        wb.Worksheets.Add(dt, "Customers")
                        wb.SaveAs(Server.MapPath("~/Files/SqlExport.xlsx"))
                    End Using
                End Using
            End Using
        End Using
    End Using
End Sub

Exported Excel