You are using Bootstrap’s framework. As do I; when I put your code into my pages, as-is, it works.
You need to add a link to Bootstrap in your head. There is nothing wrong with your code; I minimize the screen and it appears.
Seems you are missing something; view this page:
http://getbootstrap.com/getting-started/
Add this inside your head tag:
<!-- Latest compiled and minified CSS -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">
When I add a head tag and put in that code to your pen, it works.