When you upgrade your script via Fantastico, and the process does not go well, you have to restore a backup copy of your phpBB2, and then try again. The Fantastico software creates a backup before every script upgrade attempt. You can restore it as per the instructions listed in the following knowledge base article:

http://kb.siteground.com/article/How_can_I_restore_a_Fantastico_backup.html

If you are a SiteGround customer you can attempt to upgrade the script through cPanel->Softaculous or you can request an Application upgrade, by posting a ticket under:

Customer Area -> Get Support Center -> Get Support HelpDesk -> Exclusive Professional Services -> Application Upgrade