Unlock User Account (Enable User Login) on Linux

To unlock a user account that has been locked on linux, you can use the passwd command.

The command is,

sudo passwd -u username

For example, to unlock the user 'linda', so 'linda' can login to the system again,

sudo passwd -u linda

Unlocking password for user linda.
passwd: Success

Now you can try to login as 'linda' and you will see that login is working again.