Try opening the file with an account name admin and nothing in the password box. This is the default setting for any new FileMaker file.
You aren't normally asked for an account name and password on such a file because File Options is normally set to enter the admin and no password automatically. If you can get the file open, that's the next thing to check and correct.
Account Name: admin
Thank you so much. This suddenly happened to me today and it freaked me out! This got me back into my file.