Users can become locked out of using FCM after 5 failed log in attempts. When this happens, FCM will send an email to the user with a link to unlock their account. Here is a screenshot of the email below.
*Reminder to check spam/junk folders if Users do not receive this email right away.
FCM does not have the ability to Unlock Users in FCM due to security reasons. Only the Users themselves have this ability by using the link in the email sent.