Why am I unable to place one div below the other div?

I know this sounds too simple but I am unable to place one div below the other div , and my code is

html:

<div id="gamediv"></div>
<div class="style"></div>

css:

.style {
    width:728px;
    height:90px;
    margin-left:auto;
    margin-right:auto;
}