To see all jails:

fail2ban status

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Our $JAIL is usually ssh.

To see which IPs are banned currently:

fail2ban-client status ssh

If you want to unban a host banned by fail2ban on Ubuntu 14.04 or later use this method:

fail2ban-client set $JAIL unbanip $IP
#example with a jail named ssh
fail2ban-client set ssh unbanip 10.0.0.20

Please don't try to undo the ban by deleting the iptables rule. Fail2ban will recreate the rule if it is missing but fail2ban has still stored the ban.

On 12.04 hosts fail2ban is too old and doesn't know the unbanip action.

sudo iptables -F

is the only way here since sudo service fail2ban restart just re-bans the IP again after starting.