Maybe you did not set a password if you don't remember it and so leaving it blank will work?
I believe FMI, at their discretion, can remove the password from your file for a fee. There is also a for purchase windows app that does something similar.
Try Username: Admin, with no password & enter.
Else If you have saved your password to your Mac OS Keychain, you can find them from there!!
Otherwise we don't have choice! Have to contact FileMaker support and ask to see if they can help you out!!!