CGI issue.
Backing up websites is customer responsibility, a bit awkward if you didn't do one before attempting the upgrade. 
Do you have a backup of your PHPBB database tables - again just-in-case?
If you are really stuck, since PHPBB holds its data in a database, saving the configuration file, removing current files, reinstalling the required version of the program, then reinstating the configuration file and setting file permissions correctly would probably get the forum working again. Of course any add-on modules would need to be reinstalled.
However, before taking that drastic action the current problem may be relatively easy to fix - it's likely to be a file permissions issue. There's more on this in Sections 6 to 9 of the CGI Platform PHP Upgrade FAQ.
To fix file permissions I suggest using the script posted in the opening post in this topic. The script needs to be copied into a text editor (eg Notepad on Windows), saved as permissions.cgi (say) and uploaded as ASCII to your phpbb1 folder. Change its file permissions to 700 using your FTP program, then browse to the file to execute it. It should display a table showing folder and file permissions set - this is for information only.
After that completes try accessing the database update script again - hopefully it will work and the forum will be operational again.
Do you have a backup of your PHPBB database tables - again just-in-case?
If you are really stuck, since PHPBB holds its data in a database, saving the configuration file, removing current files, reinstalling the required version of the program, then reinstating the configuration file and setting file permissions correctly would probably get the forum working again. Of course any add-on modules would need to be reinstalled.
However, before taking that drastic action the current problem may be relatively easy to fix - it's likely to be a file permissions issue. There's more on this in Sections 6 to 9 of the CGI Platform PHP Upgrade FAQ.
To fix file permissions I suggest using the script posted in the opening post in this topic. The script needs to be copied into a text editor (eg Notepad on Windows), saved as permissions.cgi (say) and uploaded as ASCII to your phpbb1 folder. Change its file permissions to 700 using your FTP program, then browse to the file to execute it. It should display a table showing folder and file permissions set - this is for information only.
After that completes try accessing the database update script again - hopefully it will work and the forum will be operational again.