The answer of ukoda is wrong:
fail2ban-client without parameters and you see a list of possible commands:
get JAIL actionunban ACT
This gets the unban command for the action ACT for JAIL.
Look into the action parameter of the jail you defined, you probably have an iptables action and maybe some more like sendmail, whois or whatever. so in case your action was iptables it will look like this:
fail2ban-client get JAIL actionunban iptables
and the answer will be:
iptables -D fail2ban-NAME -s IP -j DROP
It will only show you what you would have to write for an unban. There is no unban command itself.