覆盖文字不隐藏，启动和关闭覆盖导航不工作

目前我有一个标题，子标题，基于css的菜单，然后是一个覆盖显示的文本，默认情况下应该隐藏。

我希望我的代码产生的是标题，子标题，基于css的菜单，然后是一个"打开"按钮，启动覆盖，如本例中所示。

This Fiddle给出了一个可复制的例子。

可能是我的其他一些HTML或CSS打破了覆盖，但它不清楚究竟是如何。

这是关键HTML:

<div id="mySidenav" class="sidenav">
  <a href="javascript:void(0)" class="closebtn" onclick="closeNav()">&times;</a>
  <a href="#">About</a>
  <a href="#">Services</a>
  <a href="#">Clients</a>
  <a href="#">Contact</a>
</div>
<!-- Use any element to open the sidenav -->
<span onclick="openNav()">open</span>

关键JavaScript:

/* Set the width of the side navigation to 250px */
function openNav() {
    document.getElementById("mySidenav").style.width = "250px";
}
/* Set the width of the side navigation to 0 */
function closeNav() {
    document.getElementById("mySidenav").style.width = "0";
}

和

/* The Overlay (background) */
.overlay {
    /* Height & width depends on how you want to reveal the overlay (see JS below) */   
    height: 100%;
    width: 0;
    position: fixed; /* Stay in place */
    z-index: 1; /* Sit on top */
    left: 0;
    top: 0;
    background-color: rgb(0,0,0); /* Black fallback color */
    background-color: rgba(0,0,0, 0.9); /* Black w/opacity */
    overflow-x: hidden; /* Disable horizontal scroll */
    transition: 0.5s; /* 0.5 second transition effect to slide in or slide down the overlay (height or width, depending on reveal) */
}
/* Position the content inside the overlay */
.overlay-content {
    position: relative;
    top: 25%; /* 25% from the top */
    width: 100%; /* 100% width */
    text-align: center; /* Centered text/links */
    margin-top: 30px; /* 30px top margin to avoid conflict with the close button on smaller screens */
}
/* The navigation links inside the overlay */
.overlay a {
    padding: 8px;
    text-decoration: none;
    font-size: 36px;
    color: #818181;
    display: block; /* Display block instead of inline */
    transition: 0.3s; /* Transition effects on hover (color) */
}
/* When you mouse over the navigation links, change their color */
.overlay a:hover, .overlay a:focus {
    color: #f1f1f1;
}
/* Position the close button (top right corner) */
.overlay .closebtn {
    position: absolute;
    top: 20px;
    right: 45px;
    font-size: 60px;
}
/* When the height of the screen is less than 450 pixels, change the font-size of the links and position the close button again, so they don't overlap */
@media screen and (max-height: 450px) {
    .overlay a {font-size: 20px}
    .overlay .closebtn {
        font-size: 40px;
        top: 15px;
        right: 35px;
    }
}