unable to access wpdb

My ajx.php located in root of wordpress where wp-load.php located.

my PC and Web Site has same version of MySQL . php 5.4 in PC and php 5.5 in Web Site.

Here is code of my file

<?php
error_reporting(E_ALL);
ini_set('display_errors', 1);

if(empty($_REQUEST['term'])){
echo "[ ]";
die();
}
else {
include('wp-load.php');
global $wpdb, $kommiku, $series;

$autocomplete_value = mysql_real_escape_string($_REQUEST['term']);
$table = $wpdb->prefix."comic_series";

$select = "SELECT * FROM <code>".$table."</code> WHERE title COLLATE UTF8_GENERAL_CI LIKE '$autocomplete_value%' ORDER BY <code>title</code> limit 20";
$query = mysql_query($select);

if($query === FALSE) {
    die(mysql_error()); // TODO: better error handling
}
while($row = mysql_fetch_array($query))
{
$results[] = array('title' => $row['title'] , 'slug' => $row['slug'] , 'cat' => $row['categories'] , 'img' => $row['img'] );
}
echo json_encode($results);

die();
}
?>

That code working fine in My PC with 3.9.1 version . but in Website got that error.

warning: mysql_real_escape_string(): Access denied for user ''@'localhost' (using password: NO)