There is no account name, only a password.
This is not possible with FileMaker's built in security features. For both 10 and 13 every password is paired with an account name.
If this is on a mac, did you perhaps have keychain set to open the file for you?
You might also try entering Admin with no password if the file was not password protected originally.
Might you have a hosted file with server side authentication?
I'm using a PC. Before I upgraded to 13, I would leave the account name enter and type in the password which would allow me to enter and modify the file. I'm not sure what the last statement means (hosted file with server side authentication)... the file is located on my computer not on a server. Any other ideas?
Open Edit | Preferences and note the User name shown there.
When a file is password protected, FileMaker automatically enters the specified user name as the account name in your password dialog.
Presumably, when you installed the new version, you specified a different user name during or after installation and thus no longer see the correct account name appear automatically.