give your linkbutton an id
<asp:LinkButton runat="server" OnClick="Content_Load" class="btn"
id="editlinkbutton">Edit</asp:LinkButton>
and change your code to this
protected void Content_Load(object sender, EventArgs e)
{
LinkButton editlinkbutton = sender as LinkButton;
HiddenField myhiddenfield = editlinkbutton.NamingContainer.FindControl("hiddenID") as HiddenField;
int myID = Convert.ToInt32(myhiddenfield.Value);
}
edit: maybe linkbutton doesnt have to have an id, not sure. my linkbuttons usually have id's :)