A user's account will become locked when they enter a password incorrectly 5 times or more.
To unlock the account, first locate the user and click on their username to enter the administration area.
Scroll down the page until you can see the Actions toolbox.
Click on the Unlock Account link.
The popup window should confirm that the account is locked.
Click on the Unlock button.
The account will be unlocked.
NB The user may now ask you to reset their password, see here for resetting