I have been battling with this for WAY too long for how simple it should be lol

I am trying to create a basic webpage that will check a column of a database table to see if a "UPC Code" is taken already, to prevent duplicate use. I'm fairly new at .NET programming, but have built asp pages before. I'm just stumped :/

I have a textbox that a user can type in a number. The page successfully checks to see if the textbox is blank and reports the error, and if there is a value, it's supposed to bind that textbox number to a gridview to see which Item# it's associated with. But for some reason, my code is not binding the value of the textbox to the gridview.

<%@ Page Title="Home Page" Language="vb" MasterPageFile="~/Site.Master" AutoEventWireup="false"
CodeBehind="Default.aspx.vb" Inherits="UPC_Checker._Default" %>

<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
UPC Code Checker

<asp:SqlDataSource ID="SqlDataSource1" runat="server"
ConnectionString="<%$ ConnectionStrings:FSDBGLConnectionString %>"

SelectCommand="SELECT [ItemNumber], [ItemUPC] FROM [_NoLock_FS_Item] WHERE ([ItemUPC] = @ItemUPC)">
<asp:SessionParameter Name="ItemUPC" SessionField="UPCText.Text" Type="String" />

<asp:TextBox ID="UPCText" runat="server" MaxLength="12"></asp:TextBox>

<asp:Button ID="CheckUPC" runat="server" Text="Check Availability" onclick="CheckUPC_Click" autopostback="true" />

<asp:Label ID="lblErrorMessage" runat="server" ForeColor="Red"></asp:Label>
<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False"
DataKeyNames="ItemNumber" DataSourceID="SqlDataSource1">
<asp:BoundField DataField="ItemNumber" HeaderText="ItemNumber" ReadOnly="True"
SortExpression="ItemNumber" />
<asp:BoundField DataField="ItemUPC" HeaderText="ItemUPC"
SortExpression="ItemUPC" />


Code Behind:
Imports System
Imports System.Collections.Generic
Imports System.ComponentModel
Imports System.Data
Imports System.Drawing
Imports System.Data.SqlClient
Public Class _Default
Inherits System.Web.UI.Page

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
Dim con As New SqlConnection(ConfigurationManager.ConnectionStrings("FSDBGLConnectionString").ConnectionString)
Dim cmd As SqlCommand = con.CreateCommand()

If IsPostBack Then

cmd.CommandText = "SELECT [ItemNumber], [ItemUPC] FROM [_NoLock_FS_Item] WHERE ([ItemUPC] = '" & UPCText.Text & "')"
End If
'End If
End Sub

Sub CheckUPC_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles CheckUPC.Click

If UPCText.Text = "" Then
lblErrorMessage.Text = "Please enter a UPC Code to check."

End If

End Sub
End Class