This video shows the simple steps involved in unblocking a user who has been blocked from your web site by Akeeba Admin Tools. If however the only admin user(s) of a Joomla installation are blocked and This video shows the simple steps involved in unblocking a user who has been blocked from your web site by Akeeba Admin Tools.
If however the only admin user(s) of a Joomla installation are blocked and cannot access the site, then you need to disable akeeba admin tools: https://www.akeebabackup.com/documentation/troubleshooter/atwafissues.html