Percentage measurements in CSS are relative; if .nav is inside of something only half as wide of the page, .nav will be 100% of that width - half the page. I think that’s most likely the cause, your nav is probably inside something too narrow.
Would you mind copy/pasting all of your HTML and CSS code into here so we can see it?
Here’s a post on how to format it so that it will be visible :
<!DOCTYPE html>
<html>
<head>
<link href='https://fonts.googleapis.com/css?family=Roboto:400,300,500,100' rel='stylesheet' type='text/css'>
<link rel='stylesheet' href='style.css'/>
</head>
<body>
<div class="header">
<div class="container">
<h1>INNOVATION CLOUD</h1>
<p>CONNECT YOUR IDEAS GLOBALY</p>
<a href="#" class="btn">Learn More</a>
</div>
</div>
<div class="nav">
<div class="container">
<ul>
<li>Register</li>
<li>Schedule</li>
<li>Sponsors</li>
<li>About</li>
<li>Contact</li>
</ul>
</div>
</div>
<div class="main">
<div class="container">
<img src="https://s3.amazonaws.com/codecademy-content/projects/innovation-cloud/cloud.svg">
<h2>The Innovation Cloud Conference</h2>
<p>Connect with the best minds across a wide range of industries to share ideas and brainstorm new solutions to challenging problems.</p>
<p>Hear industry leaders talk about what worked (and what didn't) so that you can save time on your most challenging projects.</p>
<p>Learn about the latest research and technologies that you can use immediately to invent the future.</p>
</div>
<div class="jumbotron">
<div class="container">
<h1>Stay Connected</h1>
<p>Receive weekly insights from industry insiders</p>
<a href="#" class="btn">Join</a>
</div>
</div>
<div class="footer">
<div class="container">
<footer>
<p> © Innovation Cloud Conference</p>
</footer>
</div>
</div>
</body>
</html>