the thumbnails of my web page can’t appear

i use mysql xampp for database and i use that database for save my thumbnails.
this my code for the dashboard buat idk for the error or maybe just require something?
please give tell me

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Dashboard</title>
  <link rel="stylesheet" href="style_dashboard.css">
</head>
<body>
  <!-- Header -->
  <header>
    <h1>Perpustakaan Online</h1>
    <nav>
      <ul>
        <li><a href="./dashboard.php">Home</a></li>
        <li><a href="profile.php">Profile</a></li>
        <li><a href="../logout.php">Logout</a></li>
      </ul>
    </nav>
  </header>

  <!-- Main Content -->
  <main>
    <h2>Dashboard Buku</h2>
    <form method="get">
      <input type="text" name="search" placeholder="Search...">
      <input type="submit" value="Search">
    </form>
    <ul class="book-list">
      <?php
        // Membuat koneksi ke database
        $host = 'localhost';
        $user = 'root';
        $password = '';
        $database = 'library_db';
        $conn = mysqli_connect($host, $user, $password, $database);

        // Memeriksa koneksi
        if (mysqli_connect_errno()) {
          echo "Failed to connect to MySQL: " . mysqli_connect_error();
          exit();
        }

        // Membuat query untuk mengambil data buku
        $query = "SELECT * FROM book";

        // Menambahkan kriteria pencarian jika ada
        if (isset($_GET['search'])) {
          $search = mysqli_real_escape_string($conn, $_GET['search']);
          $query .= " WHERE title LIKE '%$search%' OR author LIKE '%$search%'";
        }

        // Menjalankan query
        $result = mysqli_query($conn, $query);

        // Menampilkan daftar buku
        while ($row = mysqli_fetch_assoc($result)) {
          echo "<li>";
          echo "<img src='" . $row['thumbnail'] . "' alt='" . $row['title'] . "'>";
          echo "<h3>" . $row['title'] . "</h3>";
          echo "<p>Penulis: " . $row['author'] . "</p>";
          echo "</li>";
        }

        // Menutup koneksi
        mysqli_close($conn);
      ?>
    </ul>
  </main>

  <!-- Footer -->
  <footer>
    <p>&copy; 2023 Perpustakaan Online. All rights reserved.</p>
  </footer>
</body>
</html>

enter image description here

i think that from the name of picture(thumbnail) or from the database or the code and maybe from th image format