Bootstrap collapse isn’t working; hamburger menu not showing up

The markup of your navbar is not correct. The markup should be as follows:

<nav class="navbar navbar-inverse navbar-fixed-top">
    <div class="container">
        <div class="navbar-header">
            <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar" aria-expanded="false" aria-controls="navbar">
                <span class="sr-only">Toggle navigation</span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </button>
            <a class="navbar-brand" href="#">Huisje Thuisje</a>
        </div>
        <div id="navbar" class="navbar-collapse collapse">
            <ul class="nav navbar-nav navbar-right">
                <li><a href="#contact">Contact</a></li>
                <!-- Put here your menu items -->
            </ul>

        </div --><!--/.navbar-collapse -->
    </div>
</nav>

In the ul element, place the menu items. That should do the trick. See also the documentation