mysql syntax error

[Moggy]

No, i think the error is because the username is empty, try

$username = $_SESSION['username']; 
$mysqlquery = mysql_query("SELECT * FROM members WHERE username='" . $username ."'"); 
if ($mysqlquery)
$fetch_users_data = mysql_fetch_object($mysqlquery);
else
die("username=" . $username . " mysql error=" . mysql_error());