Thanks for the additional information. Safe Mode should require the password for the Microsoft account or local Windows account instead. If Safe Mode still asks for the A1B2C3 challenge phrase and PIN and does not provide a password option, please confirm whether you actually see Safe Mode displayed on the screen after startup. A screenshot of that sign-in screen would also be useful; please cover your email address or other personal information.
Since you mentioned that Sign-in options is also completely missing, if the password option cannot be obtained even in Safe Mode, return to:
Shift + Restart > Troubleshoot > Advanced options
and check whether System Restore is available. If there is a restore point from before this problem started, System Restore can roll back Windows system settings without deleting your personal files.
If the problem started immediately after a Windows update, you can also check Advanced options > Uninstall Updates and try removing the latest quality update.