How to unlock iCloud account and restore access to the owner?
How to unlock iCloud to owner
[Re-Titled by Moderator]
If you are the owner of iCloud account , and the device is with you , the original owner can unlock it , first try to reset Apple ID password with correct credentials If your Apple Account is locked, not active or disabled - Apple Support
Either the mail address or password that you are trying to sign in is incorrect as there might be typing mistakes in both , enter the mail address and password correctly , sometimes if incorrect credentials are entered multiple times server assumes some one is tricking and using phishy attempts .
Server doesn't know who is using the Mac , it assumes and locks the account see the complete details in the article If your Apple Account is locked, not active, or disabled - Apple Support
The second reason if Apple ID account doesn't exist on the iCloud and it is erased from the server How to delete your Apple Account - Apple Support
And that can be verified using this link https://iforgot.apple.com/appleid#!§ion=appleid
Enter the correct credentials if the account exists a green check mark will always appear the account is verified .
Note : if you don't remember Apple ID and its password , and Mac is in your possession better change the account to 2FA see the articles Two-factor authentication for Apple Account - Apple Support (IN)
Get a verification code and sign in with two-factor authentication - Apple Support
The secondary authentication account can be only be opened with the help of apple support advisors Contact Apple for support and service - Apple Support (IN)
( a ) If you lose answers for Apple ID provide credit card details that was added in the account .
( b ) if you have lost Apple ID password provide invoice copy of Mac to apple support advisors .
( c ) Contact apple support , they will send a link on mail account , click on link after 24 hours to reset apple id password , however the link is valid for additional 3 hours .
( d ) Support pin is used by apple support advisors to verify the account when you know apple id password Generate a temporary Support PIN - Apple Support
Note : if device is refurbished , and iCloud account is locked from the previous owner , the account can be unlocked from that person , you must be in touch with that person , only apple support can open the account , condition that person should provide account credentials to apple support .