Failing to log in on button click

My form refuses to execute a login.

I have watched tutorials, googled, tried various codes, but still no success.

Here is my code:

        private void LoginFirst_Load(object sender, EventArgs e)
        {
        }
        private void Loginbtn_Click(object sender, EventArgs e)
        {
            string s1 = Usertxt.Text;
            string s2 = Passtxt.Text;

            string connect = @"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=C:\Users\simeo\source\repos\SparesPOS\SparesPOS\PartsPosDB.mdf";

            SqlConnection conn = new SqlConnection(connect);

            conn.Open();
            SqlCommand cmd = new SqlCommand("Log", conn);
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Parameters.AddWithValue("@User", s1);
            cmd.Parameters.AddWithValue("@Pass", s2);
            cmd.ExecuteNonQuery();
            DataTable dt = new DataTable();
            SqlDataAdapter spd = new SqlDataAdapter();
            spd.SelectCommand = cmd;
            spd.Fill(dt);
            conn.Close();

            if (dt.Rows.Count > 0)
            {
                this.Hide();
                MainForm f1 = new MainForm();
                f1.Visible = true;
                this.Visible = false;
            }
            else
            {
                MessageBox.Show("failed to log in");
            }
        }