How do I unlock an user account (user login id) under Linux operating systems?

The passwd command, used for changing passwords as well used for locking and unlocking accounts on Linux and Unix-like operating systems.

Linux: Unlocking An Account Syntax

The syntax is as follows:

passwd -u username

To unlock the password of the named account called vivek, type:

passwd -u vivek

This option re-enables a password by changing the password back to its previous value (to the value before using the -l option).