Nav bar problem

[Michael Baadsgaard]

Hi i have this navbar.
But on phones it don't work. When i click on the 3 lines no menu come down.
Hope someone can help.

<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="#">Project name</a>
</div>
<div id="navbar" class="collapse navbar-collapse">
<ul class="nav navbar-nav">
<li><a href='index.php'>Forside</a></li><li><a href='tilfoj.php'>Ny opskrift</a></li> </ul>
</div><!--/.nav-collapse -->
</div>
</nav>