Forum Moderators: rogerd
You need the phpBB Admin toolkit, which is a recovery tool approved by the phpBB team and available here [phpbb.com] (choose the latest version even if it is a beta). You need FTP access to the server so you can upload the toolkit.php file, as well as the database username and password. The script runs completely independently from phpBB.
Just remember to delete the script from the server when you've finished. ;)
you could create a fresh install and create an admin user there and then export that row from mysql on the new setup and write it over the old row for the admin user.
There are a good number of solutions at a mysql level.
Is there nothing in the phpbb forums about this scenario?
<added>encyclo beat me to a better answer ;)