When you create a new file it gets a login and password by default. Did you remove those? If you did you are out of luck. If you didn't you should still be able to log in using the default login. The default login is:
The password is blank. So leave it empty.
I Just re-read your original post and you made a mistake. If you go to File-Open and enter a password there the file will be opened using the login you entered there.
But that is not the place to enter new accounts and passwords.
Before Filemaker can open your file with thal login, that login needs to exist first.
You need to create accounts in "File" - "Manage" - "Security" first. Then go to "File" - "File Options" - "Log in using..."
But there is absolutely no point in creating a login first and then having your file open automatically using that login. That way EVERYBODY can still open that file without the need to enter a password.
You should go to: "File" - "File Options" - "Log in using..." and uncheck that bos that says " Log In Using..."
That way a login and password needs to be entered before the file can be opened.
But if you don't know what you are doing in "File" - "Manage" - "Security" then please don't mess around in there. You can get into trouble if you set the wrong stuff there.