OK, here's more oddness. I went back to a snapshot we had online of the file before the power outage and it's asking me for a password too!
Are you on a Mac? If so, someone, at some time, may have stored the file's password in the Keychain. So the file seemed to open with no password. Your keychain may have been damaged, you moved to another computer, erased your drive, or created a new user, etc., thus losing the saved password.
If it's an issue with the keychain all is not lost! You could try using the Keychain app and look at the keys. Find any FileMaker keys and look for the password there by checking "show password" and entering your admin password. It could still exist but for whatever reason the OS is no longer making the association.
That's my guess anyway!
I did access it from a mac and a pc. Most recently, I've been using the pc to open it.
Any other ideas?
Do you have access to a backup file the day before the outage? I'm asking because you were able to open the file earlier in the day, and I'm assuming you closed the file properly the night before. We are assuming the damage occurred because of the power outage, but it may have been just coincidence.
If the backup from the day before also has problems, then I would uninstall and reinstall FileMaker Pro, and also run some disk utility to make sure there are no problems with the hard disk.