[SOLVED] Content flows below Navigation (menubar): responsive website

I've decide to upload my responsive website, though noticed the content (in this case the dummy text) doesn't stay below the navigation menu.

( http://www.azhir.co.uk/web_zip/default.html )

As you'll see it's a basic css style and I use css style to create the hamburger menu bar.

html, body {
          margin: 0;
          padding: 0;
          width: 100%;
    }

    body {
          font-family: "Helvetica Neue",sans-serif;
          font-weight: lighter;
    }

    header {
          width: 100%;
          height: 0.01vh;
          background-color: red;
    }

    .content {
          width: 94%;
          margin: 4em auto;
          font-size: 20px;
          line-height: 30px;
          text-align: justify;
    }

    .logo {
          line-height: 60px;
          position: fixed;
          float: left;
          margin: 16px 46px;
          color: black;
          font-weight: bold;
          font-size: 20px;
          letter-spacing: 2px;
    }

    nav {
          position: fixed;
          width: 100%;
          line-height: 60px;
    }

    nav ul {
          line-height: 60px;
          list-style: none;
          background: rgba(0, 0, 0, 0);   /*transparency rgba(0,0,0,0)*/
          overflow: hidden;
          color: #fff;
          padding: 0;
          text-align: right;
          margin: 0;
          padding-right: 40px;
          transition: 1s;
    }

    nav.black ul {
          background: white;  /*changes color of bar*/
    }

    nav ul li {
          display: inline-block;
          padding: 16px 40px;;
    }

    nav ul li a {
          text-decoration: none;
          color: black;
          font-weight: bold;
          font-size: 16px;
    }

    .menu-icon {
          line-height: 60px;
          width: 100%;
          background: white; /*changes hamburger color*/
          text-align: right;
          box-sizing: border-box;
          padding: 15px 24px;
          cursor: pointer;
          color: black;
          display: none;
    }

    @media(max-width: 786px) {

          .logo {
                position: fixed;
                top: 0;
                margin-top: 16px;
          }

          nav ul {
                max-height: 0px;
                background: white; /*mobile color*/
          }

          nav.black ul {
                background: white;
          }

          .showing {
                max-height: 34em;
          }

          nav ul li {
                box-sizing: border-box;
                width: 100%;
                padding: 24px;
                text-align: center;
          }

          .menu-icon {
                display: block;
          }

    }

This is the css for my menu.css

body{
  margin: 0;
  font-family: Arial, Helvetica, sans-serif;
  background-color: white;
}

a{
  color: black;
}

.header{
  background-color: white;
  box-shadow:1px 1px 4px 0 rgba(0,0,0,.1);
  position:fixed;
  width:100%;
  z-index:3;
}

.header ul{
  margin:0;
  padding:0;
  list-style:none;
  overflow:hidden;
  background-color:white;
}

.header ul a{
  display:block;
  padding:20px;
  boder-right:1px solid grey;
  text-decoration: none;
}

.header ul a:hover{
  background-color: #e6e6e6;
}



.header .logo{
  float:left;
  display:block;
  font-size: 2em;
  padding: 10px 20px;
  text-align: center;
  text-decoration: none;
}

.header .menu{
  clear:both;
  max-height:0;
  transition:max-height .2s ease-out;
}

/*hamburger*/

.header .menu-icon{
  padding: 28px 20px;
  position: relative;
  float:right;
  cursor:pointer;
}


.header .menu-icon .nav-icon{
  background:black;
  display: block;
  height:2px;
  width:18px;
  position:relative;
  transition: background .2s ease-out;
}

.header .menu-icon .nav-icon:before{
  background:black;
  content:"";
  display:block;
  height:100%;
  width:100%;
  position:absolute;
  transition: all .2s ease-out;
  top:5px;
}

.header .menu-icon .nav-icon:after{
  background:black;
  content:"";
  display:block;
  height:100%;
  width:100%;
  position:absolute;
  transition: all .2s ease-out;
  top:-5px;
}

.header .menu-btn{
  display: none;
}

.header .menu-btn:checked ~ .menu{
  max-height:240px;
}

.header .menu-btn:checked ~ .menu-icon .nav-icon{
  background: transparent;
}

.header .menu-btn:checked ~ .menu-icon .nav-icon:before{
  transform: rotate(-45deg);
  top:0;
}

.header .menu-btn:checked ~ .menu-icon .nav-icon:after{
  transform: rotate(45deg);
  top:0;
}

@media (min-width:48em){
  .header li{
    float:left;
  }
  .header li a{
    padding:20px 30px;
  }

  .header .menu{
    clear:none;
    float:right;
    max-height: none;
  }

  .header .menu-icon{
    display:none;
  }

  .header .logo{
    float:left;
    display:block;
    font-size: 2em;
    padding: 10px 8%;
    text-align: center;
    text-decoration: none;
  }

}

/*content*/
.content {
  width: 85%;
  font-size: 20px;
  line-height: 30px;
  text-align: inherit;
  margin-left:  auto;
  margin-right: auto;
}

and the html code is pretty basic and just;

<html>
  <head>
    <link rel="stylesheet" href="css/menu.css">
  </head>

  <body>
    <header class="header">
      <a href="/" class="logo">Azhir Mahmood</a>
      <input class="menu-btn" type="checkbox" id="menu-btn"/>
      <label class="menu-icon" for="menu-btn"><span class="nav-icon"></span></label>
      <ul class="menu">
        <li><a href="#ALPHA">ALPHA</a></li>
        <li><a href="#BETA">BETA</a></li>
        <li><a href="#GAMMA">GAMMA</a></li>
      </ul>
    </header>

    <div class="content">
          <p>
                Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.
          </p>
          <p>

  </body>
</html>

Content flows below Navigation (menubar): responsive website

Why