var topNav=document.getElementByClassName("topnav");  中 Element 应该是 Elements

 <script type="text/javascript">
        window.onload=function(){
            var topNav=document.getElementById("Topnav");
            var top_Nav=topNav.getElementsByTagName("li");
            for(i=0;i<top_Nav.length;i++){
                top_Nav[i].onmouseover=function(){
                var top = this.getElementsByTagName("ul")[0];
                top.style.display="block";
                }
            }
            
        }

    </script>

用firebug 看看你报错的地方