If you are dealing with a closed database and TTK is giving you an invalid username/password error, try this KB article:
http://partners.retailpro.com/Support/Knowledgebase.php?a=145
First, check the alert logs in the trace folder. If you are getting Oracle 600 error, follow the above KB.
Basically, the file in the IR folder goes correupt. Renaming and restarting the DB should fix it.