MySQL blocks clients which error made while connecting to protect MySQL from malformed client.
So first, you need to find what sort of error is....
You might check MySQL error log in data directory. (typically hostname.err)
Or, you can increase
max_connect_errors (what is current value?) maximum value depends on architecture. on 32 bit, 4294967295. 18446744073709547520 for 64 bit. (Manual)
mysql> SET GLOBAL max_connect_errors = 100000000;
But this is not real solution if error is frequently occurred.
FLUSH HOSTS can help you to eliminate blocked host right now.
mysql> FLUSH HOSTS;
If want to run from outside mysql console then use mysqladmin command:
# mysqladmin flush-hosts