mysql_fetch_array() expects parameter 1 to be resource, boolean given in

<?php
session_start();
include_once 'dbconnect.php';

if(isset($_SESSION['administrator'])!="")
{
	header("Location: administrator.php");
}

if(isset($_POST['Submit']))
{
	$username = mysql_real_escape_string($_POST['username']);
	$upass = mysql_real_escape_string($_POST['pass']);
	$res=mysql_query("SELECT * FROM account WHERE username='$username'");
	$row=mysql_fetch_array($res);
	$coop=$row['coop'];
	
	if($row['password']==base64_encode($upass))
	{
		$_SESSION['administrator'] = $row['user_id'];
		if($coop=="YES")
		{
		header("Location: administrator.php");
		}
		else
		{
			?>
        	<script>alert('Account Disallow on system');</script>
        	<?
		}
		
	}
	else
	{
		?>
        <script>alert('Incorrect User and Password');</script>
        <?
	}	
}
?>