Drupal - How to unblock my own admin account after 5 failed login attempts?

To answer the question in your title (which appears to become more complicated following your actual question), you just need to clear the flood table in your database. If you're running Drush, then the following should sort you out:

drush php-eval 'db_query("DELETE FROM flood");'

Failing that, just clear the flood table from your database manually :)

Clearing the flood table will give the OP another 5 tries guessing the password. (And if he has messed with the password field directly in MySQL, the passord field may have become munged and therefore useless.)

Clearing the flood table will not allow him to login as admin, which is what I assume the OP actually want to do.

However, there are a couple of other drush commands that may be handy. The first will produce a one time password reset link for uid 1, the second sets a new password for user admin:

drush uli
drush upwd admin --password="newpassword"

If you are not much experienced with Drush kind of stuff, you can run the following sql in your phpmyadmin

DELETE FROM flood;

The same error can be happened if you tried same user name with 5 or more times