In the empty fields try to enter your I cloud id & password .
Have you turned on file vault ?

Also as per article : macOS Sierra: Encrypt the contents of your Mac with FileVault
WARNING: Don’t forget your recovery key. If you turn on FileVault and then forget your login password and can’t reset it, and you also forget your recovery key, you won’t be able to log in and your files and settings will be lost forever.
And another method is there to reset the password :
How to reset the admin password , if one has forgotten it .
Close all applications & shut down the computer , and plug the charger .
Press command + R + power key simultaneously , don't release the keys until apple logo appears , the bar will load slowly , let it be completed after a while you will enter in recovery mode .if you have enabled firmware password , turn it off .
Take your cursor on top menu bar & click on utilities > open terminal from here & type resetpassword the word is single & no space , all alphabets are small .Click on enter & close the terminal window .
On the screen you will see user name of the account , just select it ( press forcefully on it to get selected ) > click on next on the bottom side .
A rectangular bar appears > enter your new password carefully and click on “ SAVE “. And again go to top menu bar you will see apple logo > click on restart .