That does sound very upsetting. Do you have a downloaded backup? That would always be the best way to recover from a disaster. Ideally you also make sure that your latest backup is kept in two safe places.
If you don’t have a backup in your hands, then it might be, sadly, that your only hope is copying data from any accidental online archives, whether at archive.org or in Google’s caches, or elsewhere.
For Hetzner to help you, they would need to know that you are indeed the original owner of the site, and that the email modification was malicious. That might be difficult for you to demonstrate. They would also need to have the forum data or backup, and they might not have that any more.