responsive menu not showing

Question

I want to show the menu items as seperate rows on smaller screen. This is pretty basic and seen many places. The effect i am going for is:

With what i have so far (code Below) the menu items just show up next to the logo (thats how they are there for desktop).

Also i need to add a Background Image to the menus items. One image that fills the entire area and not the same image repeating for each menu

I have already added:

<meta name="viewport" content="width=device-width, initial-scale=1">

Here is my fiddle

//make the topnav responsive for mobile screens
function topNav() {
    var x = document.getElementById("myTopnav");
    if (x.className === "myTopnav") {
        x.className += " responsive";
    } else {
        x.className = "myTopnav";
    }
}

.topnav {
    background-repeat:no-repeat;
    background-size:cover;
    overflow:visible;
    width: 100%;
    padding-bottom: 0px;
}

.topnav nav{
    padding-top: 5px;
    padding-bottom: 8px;
    background: -webkit-linear-gradient(left, transparent, black, black, transparent); /* For Safari 5.1 to 6.0 */
    background: -o-linear-gradient(left, transparent, black,black, transparent); /* For Opera 11.1 to 12.0 */
    background: -moz-linear-gradient(left, transparent, black,black, transparent); /* For Fx 3.6 to 15 */
    background: linear-gradient(to right, transparent, black, black, transparent); /* Standard syntax (must be last) */
    width:100%;
}

.topnav nav div{
    width: 85%;
    margin-left: 7.5%;
    margin-right: 7.5% ;
    display:flex;
    align-items: center;
    justify-content: center;
}

.link {
    color: white;
    text-decoration: none;
    font-size: 11px;
    font-weight: 700; /* this is futura heavy*/
    width:11.25%;
    letter-spacing: 2px;
    word-wrap:true;
    display:inline-block;
    vertical-align: top;
    text-align: center;
    /*line-height: 5em;*/
}

.notlink {
    width:30.4%;
    margin-left: 1%;
    margin-right: 1%;
    display: inline-block;
}

.notlink img {
    width: 100%;
}

.topnav nav .link:hover {
    color: #008ed2;
}

.icon {
    display: none;
    font-size: 32px;
    color: white;
    text-decoration: none;
    text-align: center;
    line-height: 40px;
    min-width: 40px;
    min-height: 40px;
}


/* When the screen is less than 600 pixels wide, hide all links, except for the first one ("Home"). Show the link that contains should open and close the topnav (.icon) */
@media only screen and (max-width: 767px) {
    .myTopnav a.link {display: none;}
    
    .topnav nav div{
        width: 100%;
        margin-left: 0;
        margin-right: 0;
        position: relative;
        top: 0;
        left: 0;
    }
    
    .icon {
      display :inline-block;
      position : absolute;
      right : 0;
    }
    
    .notlink {
        width: 70%;
        display : flex;
        align-items: center;
        justify-content : center;
    }
    .notlink img {
        max-width:100%;
        max-height:75px;
        width: auto;
        height: auto;
    }
}

@media screen and (max-width: 767px) {
    .myTopnav.responsive {position: relative;}
    .myTopnav.responsive .icon {
        position: absolute;
        right: 0;
        top: 0;
    }
    .myTopnav.responsive a {
        float: none;
        display: block;
        text-align: left;
    }

}

<div class="topnav">
                <nav>
                    <div class="myTopnav" id="myTopnav">
                        <a class="link" href="index.html">HOME</a>
                        <a class="link" id ="selected" href="about.html">ABOUT</a>
                        <a class="link" href="expertise.html">EXPERTISE</a>
                        <a class="notlink" href="index.html"><img src="https://www.google.com/logos/doodles/2017/earth-day-2017-5650394117046272-s.png"></a>
                        <a class="link" href="industries.html">INDUSTRIES</a>
                        <a class="link" href="friends.html">FRIENDS OF MARY</a>
                        <a class="link" href="contact.html">CONTACT</a>
                        <a href="javascript:void(0);" class="icon" onclick="topNav()">&#9776;</a>
                    </div>
                </nav>
            </div>

Answer 1

Here is the fiddle

You need to add flex-direction:column for topbar inorder to show it as column for mobile devices.

    .topnav nav div{
      flex-direction:column;
    }
    .myTopnav.responsive a.link:first-child{
      margin-top:30px;
    }
    .myTopnav.responsive a.link{
      width : 100%;
    }
    .myTopnav.responsive a.notlink{
          width: 70%;
    position: absolute;
    top: 0px;
    text-align: center;
    }

Answer 2

Here my solution (changed markup, js and css):

HTML:

<div class="topnav" data-mobile="false">
  <nav>
    <div class="myTopnav" id="myTopnav">
      <a class="link" href="index.html">HOME</a>
      <a class="link" id ="selected" href="about.html">ABOUT</a>
      <a class="link" href="expertise.html">EXPERTISE</a>
      <a class="notlink" href="index.html"><img src="https://www.google.com/logos/doodles/2017/earth-day-2017-5650394117046272-s.png"></a>
      <a class="link" href="industries.html">INDUSTRIES</a>
      <a class="link" href="friends.html">FRIENDS OF MARY</a>
      <a class="link" href="contact.html">CONTACT</a>
      <a href="javascript:void(0);" class="icon" onclick="topNav()">&#9776;</a>
    </div>
  </nav>

  <nav id="mobile-menu">
      <a class="link" href="index.html">HOME</a>
      <a class="link" id ="selected" href="about.html">ABOUT</a>
      <a class="link" href="expertise.html">EXPERTISE</a>
      <a class="link" href="industries.html">INDUSTRIES</a>
      <a class="link" href="friends.html">FRIENDS OF MARY</a>
      <a class="link" href="contact.html">CONTACT</a>
  </nav>
</div>

JS:

function topNav() {
  var topNav = document.querySelector('.topnav');
  topNav.dataset.mobile = (topNav.dataset.mobile == "false") ? true : false;
}

CSS:

html, body {
  height : 100%;
  margin : 0;
  padding : 0;
  background-color : white;
}

.topnav {
  overflow:hidden;
  width: 100%;
  padding-bottom: 0px;
}

.topnav nav:first-of-type{
  padding-top: 5px;
  padding-bottom: 8px;
  background: -webkit-linear-gradient(left, transparent, black, black, transparent); /* For Safari 5.1 to 6.0 */
  background: -o-linear-gradient(left, transparent, black,black, transparent); /* For Opera 11.1 to 12.0 */
  background: -moz-linear-gradient(left, transparent, black,black, transparent); /* For Fx 3.6 to 15 */
  background: linear-gradient(to right, transparent, black, black, transparent); /* Standard syntax (must be last) */
  width:100%;
}

.topnav .myTopnav{
  width: 85%;
  margin-left: 7.5%;
  margin-right: 7.5% ;
  display:flex;
  align-items: center;
  justify-content: center;
}

.link {
  color: white;
  text-decoration: none;
  font-size: 11px;
  font-weight: 700; /* this is futura heavy*/
  width:11.25%;
  letter-spacing: 2px;
  word-wrap:true;
  display:inline-block;
  vertical-align: top;
  text-align: center;
  /*line-height: 5em;*/
}

.notlink {
  width:30.4%;
  margin-left: 1%;
  margin-right: 1%;
  display: inline-block;
}

.notlink img {
  width: 100%;
}

.topnav nav .link:hover {
  color: #008ed2;
}

.icon {
  display: none;
  font-size: 32px;
  color: white;
  text-decoration: none;
  text-align: center;
  line-height: 40px;
  min-width: 40px;
  min-height: 40px;
}

#mobile-menu {
  width : 100%;
  height : 100%;
  padding : 20px;
  background: url(https://pbs.twimg.com/profile_images/638771889339404289/h64dZ4wh_400x400.png) no-repeat center center fixed;
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
  display: flex;
  flex-direction: column;
  /*justify-content: center; --> add this to center vertically*/ 
  align-items: center;
  box-sizing : border-box;
}

#mobile-menu > * {
  color : black;
}

.topnav[data-mobile=false] #mobile-menu {
  display : none;
}


@media only screen and (max-width: 767px) {
  .myTopnav a.link {
    display: none;
  }

  .topnav[data-mobile=true]{
    height : 100%;
  }

  .topnav nav div{
    width: 100%;
    height : 100%;
    margin-left: 0;
    margin-right: 0;
    position: relative;
    top: 0;
    left: 0;
  }

  .icon {
    display :inline-block;
    position : absolute;
    right : 0;
  }

  .notlink {
    width: 70%;
    display : flex;
    align-items: center;
    justify-content : center;
  }

  .notlink img {
    max-width:100%;
    max-height:75px;
    width: auto;
    height: auto;
  }
}

After this you need to place it correctly over your following content when the mobile menu is expanded.

fiddle