IntegraMOD

Are you locked out of the account, do you get a banned or error message?

One of the 'easiest' ways is to reinstall!

Failing that,
Register a new user on your site with a different username but the same password you wish to have as admin,
Then open up phpmyadmin, in the left column, select the db of your install, then scroll down to phpbb_users (or whatever db prefix instead of phpbb_).
Now click on the top tab Browse.
You should have 3 rows:- the first is a guest user, the second is your premier admin account, and third is the account you just made.
Check that the second row has the admin username that you expected. If it is correct, look in the user password row and copy the encrypted password in the 3rd row. (the 2nd and 3rd row encrypted strings should look different)
Now on the 2nd row, click on the pencil to edit that row.
If the username was not as you expected, you can replace it here.
Go to the user_password cell and paste over the contents with the string you had copied.
Click go.

If you exceeded the number of user login attempts, go back to editting row2, scroll to the near end of all the fields and change user_login_attempts to 0 and click go.
Now you should be able to login!