When sites were moved from 1.0 to 2.0 I noticed that instead of yourdomain as the username for the database, the new database has a username root. (With a different password)
There are two ways to solve the problem. Edit the phpBB setup to use root and the newpassword as the login/password information - or add a user to the database matching the old information (mysql user and password) The easy route to adding the new user is to login to the site manager and install phpmyadmin (should be under site applications/database), then work from there to add another user to your database. (If you set the username and password to what you had before that should get you there, although I haven't worked with phpBB setup...)
Avery