Problem
Flush the cPHulk brute force filter table on cPanel provided you still have access to SSH.
tl;dr
Run this command from bash:
echo "delete from brutes; delete from logins;" | mysql cphulkd
Solution
cPHulk is cPanel's built in brute force firewall/IP blocker, extremely similar to fail2ban in its functionality. It automatically detects numerous failed login attempts, and blocks likely brute force attacks. If an IP is blocked, both SSH and cPanel WHM logins will receive an "access denied" message, even if the password you are entering is correct.
If you still have SSH access (such as, from a different IP address, such as your phone's mobile hotspot, or a different office), you may attempt the above command manually. Otherwise, you may need to get KVM access to your server.
In either case, enter this command to flush the blacklisted IP addresses and permit yourself to login again:
echo "delete from brutes; delete from logins;" | mysql cphulkd