It sounds like the permissions are incorrect.
There is an article here about it
http://www.phpbb.com/kb/article.php?article_id=22
There are a number of files that require chmodded in phpBB to work correctly:
config.php: Chmod this to 777 BEFORE you start the install process. Once install is complete, chmod it back to 644.
By Default, All PHP Files should be chmod 644
/images/avatars: This is a folder that you have to chmod. This is so that, if you choose to enable avatar uploading, they can be uploaded properly. This folder must be chmodded to 777.
The easiest no fail way is to reinstall.
1) Backup your site.
2) Reinstall PHPBB
3) Restore the backup. If im right the filepermissions will stay in tact and it should start to work again.