There is a site in the mobile version of which the menu is not revealed. Ie when you click on a button, the list should open, the menu button itself is there, but nothing happens when you click. 
Here is the code itself
<div class="container-fluid"> <div class="row"> <div class="col-xs-12"> <div class="header__logo"> <a href="index.html" class="logo"> <img src="images/LOGO.png" style="max-height: 40px;" alt="Logo" > </a> </div> <div class="navbar"> <div class="yamm"> <div class="navbar-header hidden-md hidden-lg hidden-sm"> <button type="button" data-toggle="collapse" data-target="#navbar-collapse-1" class="navbar-toggle"><span class="icon-bar"></span><span class="icon-bar"></span><span class="icon-bar"></span> </button> <a href="index.html#" class="navbar-brand"></a> </div> <nav id="navbar-collapse-1" class="navbar-collapse collapse"> <ul class="nav"> <li class="first"><a href="" >Главная</a></li> <li class="first"><a href="#about" class="about">О нас</a></li> <li class=" dropdown"> <a href="" class="dropdown-toggle nav-down" data-toggle="dropdown" title="Услуги"> <span onclick="location.href=''">Услуги<b class="caret"></b></a> </li> <li class="last"><a href="#contacts" class="contacts">Контакты</a></li> </ul> </nav> </div> </div> </div> </div> </div> 
Navbaritself - getbootstrap.com/docs/4.1/components/navbar . Depending on the version you have connected, select the one you need there. - Bert