First, we must assume that you are currently storing the 
username/login, password and the email address in a mySQL table.  Once 
we know the username/login, we simply use that to locate the password 
and utilize the mail() function to send it.  

Your snippet would be something like this:

  $result = mysql_query ("SELECT password,email FROM yourtable 
                          WHERE username = '$username'");
  if ($myrow = mysql_fetch_array($result)) {
    $mailaddress = $myrow["email"];
    $subject     = "Important Message"; // don't say password in subject
    $message     = "Your password is: " . $myrow["password"] ;
    $header      = "Ancillary message or other stuff";
    mail($mailaddress,$subject,$message,$header);
  } 

Hope this is helpful.