It is not asking for you user name. It's asking for your account name--a picky but important detail that sometimes results in confusion due to FilMaker automatically entering the user name (which is specified in Preferences) as your default account name in the password dialog.
Try opening your file with admin as the account name and leave the password box empty. If that opens your file, go to File Options and reset the "open file using" portion to open your file with admin and no password--the default settings on files that are not password protected.
Was your original file protected with a password? If so use the original account name and password that opened the original file to open the converted copy.
There are two different features that could hide the fact that your original file was protected by a password:
on Mac systems, a "keychain" can include the account name and password used to open the file and thus the file opens without asking for a password.
On both Mac and Windows, the settings in File Options can specify that the file open automatically with an account name and password specified in that File Options dialog. If this is the case, opening your original file while holding down the option (mac) or shift (windows) key will cause fileMaker to ask for a password.