As long as the board is not disabled, you can request a new password on the login page with the "I forgot my password" link. You will be sent an email with a new password and a link to activate your account. Click the link to activate your account then you can use the password sent to you to login.
If your board is still disabled, go to MD5 Encryption Tool
and enter your desired password.
Copy the MD5 hash.
Using phpMyAdmin go to the phpBB3 users table
Find the row with your username and edit the row
Enter the copied MD5 has intot he user_password field and Submit.
You should now be able to log in.