notes

Keep trying passwords until you remember it. There is no other way to open a locked note. The first time you locked a note you were informed that you must remember the password in order to unlock it.

See How to lock notes on your iPhone, iPad, and iPod touch - Apple Support, particularly the "If you forgot your notes password" portion.

It always had a passcode. Face ID is supplemental to the passcode you set to encrypt the notes. If you can't remember the passcode, the notes are unreadable. You can change the passcode for encrypting new notes, but that does not change the passcode for encrypted notes that already exist.