Admin users can be locked out of their account if they try to login with the wrong password more than eight times.
Admin accounts can be unlocked by site admins from within Constituent Manager. Find the user's profile, click on the "Account" tab, and look for the "Unlock User" link at the bottom of the page.
A confirmation window will pop up, and refreshing the Account tab will show that the "Unlock User" link has disappeared.
Unlocking accounts from Admin Users
Site admin accounts can also be unlocked from the Admin Users screen, found on the "People" menu in the Composer navigation. Find the locked account in the "Group Users" section, click the arrow button next to their name, and then click the "Unlock User" link.