How to unlock linux user after too many failed login attempts

If you still have access to the system by another account
root or some other account with root-like (sudo?) privileges

1. You can view the failed logins with faillock

   # faillock --user myUsername
   myUsername:
   When            Type     Source     Valid
   Timestamp 1     TTY      /dev/tty1  V
   Timestamp 2     TTY      /dev/tty1  V
   Timestamp 3     TTY      /dev/tty1  V
   

2. Reset them with the --reset-flag

   # faillock --user myUsername --reset
   # faillock --user myUsername
   myUsername:
   When            Type     Source     Valid
   

If you've found another way to access the file system
through a rescue disk or someone else's computer

1. Navigate to /var/run/faillock (*), this folder should contain a file with the locked username

   # ls /var/run/faillock
   myUsername
   

2. Remove the file with the username to unlock

   # rm /var/run/faillock/myUsername

* This is called 'tally directory' and its location depends on your system's configuration, which is likely to be found at /etc/security/faillock.conf

"sudo" and "faillock" - How to handle "The account is locked due to X failed logins."

If you still have access with root-like privileges, you can view the failed logins...

faillock --user [USERNAME]

... and/or reset attempts status...

faillock --user [USERNAME] --reset

PLUS:

You can change the possible attempts number before a lock by uncommenting and changing the parameter...

# deny = 3

... in the file...

vi /etc/security/faillock.conf

Normally faillock automatically unlocks the account after 10 minutes (600 seconds), but you can change that too...

# unlock_time = 600

[Ref(s).: https://forum.manjaro.org/t/how-to-remove-or-increase-password-attempts/47986/2?u=eduardolucioac ]

By default tally log is present in /var/run/faillock. If it is configured in /etc/security/faillock.conf to write in different path, then use below command.

faillock --reset --user <user> --dir <tally dir path>