There likely was a password on the old file but you had it stored in your Apple keychain. QuickBooks Mac 2016 would just log you in automatically without prompting. QuickBooks Mac 2019 requires you to type the password in (though it will autofill it from the keychain if it can find it).
Have you changed machines? That's usually how the keychain loses the password. You can open Apple's Keychain Access program (use Spotlight to find it). If you were on another machine or user account when using 2016 make sure to do this there.
Then in the top right type the word QuickBooks in the search bar. You hopefully will find an entry for the QuickBooks User. Double click on that and it will prompt you for your login password (the password you use to login into your Mac OS user account). Type in your password. It should then reveal the user password you need to enter in QuickBooks.
If none of that works you can follow the flow for "forgot my password" or "reset password" on the log in screen.