It's been 2 months, but did you are anyone else make any changes in Access privileges?
Do you have any back up copies you can use, perhaps by saving a clone of the back up and importing the data from your current copy?
All we can tell from here is that Filemaker is not accepting the combination of account name and password that you are using, there's no real way we can tell why it is not. All we can do is offer a few guesses.
Passwords are case sensitive. Is the caps lock key down or is there any chance that you are not capitalizing the password correctly?
I think it happened when we moved all files from the PowerBook Mac onto the new iMac.
During the install process a user name is set up for your installation of FileMaker. Is there any chance that you now have a new user name? This user name will be entered as the default account name in the log in dialog so it is at least a possibility that this change means you are entering the correct password but that the default account name (that user name) needs to be changed to a different name before you can open the file with full access permissions.