Installatron is supposed to be making a backup of your site files and database before attempting any upgrades. If the upgrade didn't go as planned, their intention is that you can simply restore the automatic backup that was made; or that Installatron would have already restored the backup upon detecting that the installation failed.
But your hosting provider (the one selling you Installatron service, as well as your cPanel-based hosting) is who you need to talk to about this, because the installation that has been attempted & failed is using an Installatron-modified and Installatron-optimized version of the phpBB installation files. So your hosting provider's Installatron support is who needs to help you with what might have gone wrong with their custom installer / custom updater.
So if your urgent need is to get the board back online, I recommend achieving that by talking to your hosting provider about getting your site restored back to how it was before the upgrade was attempted. And then while you have them on the phone, see if they can tell from their logs why the Installatron-managed upgrade did not succeed.
When you're ready to attempt the upgrade again, you "can" just do it manually using the official phpBB 3.2.4 files downloaded from here on phpbb.com. (And then if something went wrong, be able to come here and share details for additional help.) But if you're already using and paying for the Installatron service, I would just stick with that and have them resolve why their automated service isn't living up to its promises.