on onePress when I set container to position fixed it is all disappearing?

Hello,

I am trying from this one pager theme (onePress) style on my own desires .

this is the html code :

<div id="flex-container">
	
		

    <section class="logo"><h3>hello
		
		</h3>   
    </section>


    
    <section class="menu-section">
   <div class="menu">
        <ul>
          <li><a >Home</a></li>
          <li><a >Over ons</a></li>
          <li><a >Webdesign</a></li>
          <li><a >One Pager</a></li>
          <li><a >Contact</a></li>
        </ul>
      </div>
    </section>
  
	</div>

and this is CSS code:

<div id="flex-container">
	
		

    <section class="logo"><h3>hello
		
		</h3>   
    </section>


    
    <section class="menu-section">
   <div class="menu">
        <ul>
          <li><a >Home</a></li>
          <li><a >Over ons</a></li>
          <li><a >Webdesign</a></li>
          <li><a >One Pager</a></li>
          <li><a >Contact</a></li>
        </ul>
      </div>
    </section>
  
	</div>

the problem is when I insert:

position:fiexed;

all the header is disappearing. and when I remove the position then the container is not fixed.

thanks

The page I need help with: [log in to see the link]