try this,
<div id="content" runat="server">
</div>
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
content.Visible = false;
}
}
Or
If your div is inside GridView or Repeater or any control,Then you can find it like this,
HtmlGenericControl InnerDiv = (HtmlGenericControl)e.Item.FindControl("divName");
I hope this will help you.