It is common that the IP must be blocked after 5 or more failed login attempts in Drupal.This can be overcome by following these simple steps. 

1. Login to php myadmin. 
2. Find the database of the site that has been blocked.
3. Open the “flood” table in the Database.
4. Delete the failed login attempts from the Database of your particular IP.

Now You can reattempt to login without any Problems.