Do you have cPHulk Brute Force Protection enabled? It shouldn't only be denying access to one site and would the whole server, though, so that's a bit confusing if it were the cause. If you have WHM > cPHulk Brute Force Protection enabled, you can temporarily disable it to see if that works to see if that is the cause.
Otherwise, do you have any other scripts that you've installed onto the machine? You could still check your IP if it might be listed in the firewall directly:
Where IP# is the IP number for the machine that is blocked. Also, just to make sure, please do check your IP at What's My IP Address? Networking Tools & More in case you believe that computer is coming from an IP that might not be the actual IP. This way you can ensure you are checking for the right IP.
/sbin/iptables -n -L | grep IP#