Remember everyone, when you change anything related to cookies its a good idea to clear your cookies (or in the least the cookies for said site). Using tools.php was a good idea if you're ever unsure but if all else fails you can run a query to reset any users password:
Code:
UPDATE `user`
SET `password` = MD5(CONCAT(MD5('newpassword'), `user`.`salt`))
WHERE `userid` = 1
Of course change newpassword to the actual password they wish to use and the 1 to the userid of the user in question. If your database uses a prefix then include the prefix.