使用flex的响应式导航栏没有折叠

<div class="navbar">
  <div class="menu-btn"></div>
  <ul class="nav-items">
    <li><a href="#">Home</a></li>
    <li><a href="#">About</a></li>
    <li><a href="#">Services</a></li>
    <li><a href="#">Contact</a></li>
  </ul>
</div>

.navbar {
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.nav-items {
  display: flex;
  list-style: none;
}

.nav-items li {
  margin-right: 10px;
}

.menu-btn {
  display: none;
}

@media (max-width: 768px) {
  .navbar {
    flex-direction: column;
    height: 50px;
  }

  .nav-items {
    display: none;
  }

  .menu-btn {
    display: block;
    width: 30px;
    height: 30px;
    background-color: #000;
    cursor: pointer;
  }
}