What is the reason of Host 'host_name' is blocked in MySQL?
At this point, you may need to run FLUSH HOSTS and see if this unblocks.

The MySQL Documentation says this about FLUSH HOSTS:


  
  Empties the host cache tables. You should flush the host tables if
  some of your hosts change IP address or if you get the error message
  Host 'host_name' is blocked. When more than max_connect_errors errors
  occur successively for a given host while connecting to the MySQL
  server, MySQL assumes that something is wrong and blocks the host from
  further connection requests. Flushing the host tables enables further
  connection attempts from the host. See Section C.5.2.6, “Host
  'host_name' is blocked”. You can start mysqld with
  --max_connect_errors=999999999 to avoid this error message.
  


Why should a host get blocked to begin with ??

According to the MySQL Documentation:


  If you get the following error, it means that mysqld has received many
  connect requests from the host 'host_name' that have been interrupted
  in the middle:  Host 'host_name' is blocked because of many connection
  errors. Unblock with 'mysqladmin flush-hosts'   The number of
  interrupted connect requests permitted is determined by the value of
  the max_connect_errors system variable. After max_connect_errors
  failed requests, mysqld assumes that something is wrong (for example,
  that someone is trying to break in), and blocks the host from further
  connections until you execute a mysqladmin flush-hosts command or
  issue a FLUSH HOSTS statement. See Section 5.1.3, “Server System
  Variables”. 
  
  By default, mysqld blocks a host after 10 connection errors. You can
  adjust the value by starting the server like this:


shell&gt; mysqld_safe --max_connect_errors=10000 &amp;



  If you get this error message for a given host, you should first
  verify that there isn't anything wrong with TCP/IP connections from
  that host. If you are having network problems, it does you no good to
  increase the value of the max_connect_errors variable.