Check if IP is blocked:
# iptables -L -n --line | grep [IP Address]

If IP appear as DROP or REJECT, the IP has been blocked

Unblock the IP Address:
# iptables -I INPUT -s [IP Address] -j ACCEPT

Blocking back an IP Address:
# iptables -A INPUT -d [IP Address] -j DROP

Note: Full path to IPTABLES is /sbin/iptables, you can replace command # iptables with /sbin/iptables