Older versions of FileMaker didn't use account names for access control, just passwords, so when you upgrade an old database to FileMaker 7 or beyond, FileMaker creates a dummy account. The following article may help:
However, I'm not convinced this information is entirely correct - I ran into a similar problem to you earlier today with a FileMaker 6 database that had been converted to FileMaker 10, and we found that we could logon using a blank account name and one of the passwords on the original FileMaker 6 database.
So try leaving the account name blank and entering the password, and if there's no password, try leaving that blank too, and if that doesn't work, read what the above article has to say about Guest accounts, and see if that works.
Newly converted 5.5 files with defined passwords can be opened by entering the password twice, once in the account name field and once in password field. Of course that doesn't help you if you don't know what password to enter.
Yes, that's what the FAQ about file conversion says - an account is created for each password, with a name that matches the password. However, in our case, we found that leaving the account name blank worked just as well - we didn't need to try the password/password login. Indeed, I only found the FAQ afterwards, when I was looking for documentation that confirmed how FileMaker dealt with passwords when upgrading between versions.
Of course, it's possible that the blank account name only gets you onto your database with some sort of guest access - I didn't check whether write access worked. However, it wasn't clear from the FAQ that blank accounts would be created in this way if you already had passwords on the database, which is why I'm a bit surprised that the blank account worked at all, and wondered if the FAQ was correct.
BTW, if Mannie's database really does have a password that nobody can remember, then there are password recovery programs available for FileMaker Pro...
Never thought to try a blank account name on the converted file. Interestingly enough, I went to a back up copy I made of a file immediately after conversion and sure enough, it does open when I enter the original password and a blank account name.