Registration Implementation asp.net Web Form

Question

this is my class code

namespace SECODMS.Model {
public class ApplicationUser {
    public string UserName { get ; set; }
    public string FirstName{ get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
    public string AvatarUrl { get; set; }
}

public static class AuthHelper {
    public static bool SignIn(string userName, string password) {
        HttpContext.Current.Session["User"] = CreateDefualtUser();
       // Mock user data
        return true;
    }
    public static void SignOut() {
        HttpContext.Current.Session["User"] = null;
    }
    public static bool IsAuthenticated() {
        return GetLoggedInUserInfo() != null;
    }
    public static ApplicationUser GetLoggedInUserInfo()
    {
        return HttpContext.Current.Session["User"] as ApplicationUser;
    }
    private static ApplicationUser CreateDefualtUser() {
        return new ApplicationUser {

            UserName ="vvnbnbnb",
            FirstName = "Julia",
            LastName = "Bell",
            Email = "[email protected]",
            AvatarUrl = "~/Content/Photo/Julia_Bell.jpg"
        };
    }
}

using System;
using DevExpress.Web;
using SECODMS.Model;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

namespace SECODMS {
  public partial class SignInModule: System.Web.UI.Page

  {
    protected void Page_Load(object sender, EventArgs e)
    {
    }
    protected void SignInButton_Click(object sender, EventArgs e) {

      FormLayout.FindItemOrGroupByName("GeneralError").Visible = false;

      if (ASPxEdit.ValidateEditorsInContainer(this)) {
        // DXCOMMENT: You Authentication logic  //
        string mainconn = ConfigurationManager.ConnectionStrings["SECODMSConnectionString"].ConnectionString;
        SqlConnection sqlconn = new SqlConnection(mainconn);
        string sqlquery = "Select * from Users where Username=@Username and Password=@Password";
        SqlCommand sqlcomm = new SqlCommand(sqlquery, sqlconn);
        sqlconn.Open();
        sqlcomm.Parameters.AddWithValue("@Username", UserNameTextBox.Text);
        sqlcomm.Parameters.AddWithValue("@Password", PasswordButtonEdit.Text);
        SqlDataAdapter sda = new SqlDataAdapter(sqlcomm);
        DataTable dt = new DataTable();
        sda.Fill(dt);
        sqlcomm.ExecuteNonQuery();
        if (dt.Rows.Count > 0) {
          Session["user"] = UserNameTextBox;
          Response.Redirect("~/Default.aspx");
        }
        else {}
        if (!AuthHelper.SignIn(UserNameTextBox.Text, PasswordButtonEdit.Text)) {
          GeneralErrorDiv.InnerText = "Invalid login attempt.";
          FormLayout.FindItemOrGroupByName("GeneralError").Visible = true;

        }
        else Response.Redirect("~/Default.aspx");
      }
    }
  }
}

and this my sign in code kindly advise me what's wrong I am not able to, getting the value from the session and then casting as ApplicationUser.