Comment by leakycap

User-uploads are the #1 cause of this if the developer is not doing anything otherwise against TOS. Users will upload things you don't expect in ways you don't program for. And the files they upload can be immediately spotted by any big storage provider due to the known file hashes.

So, when you do relaunch with another service provider, I recommend putting all user-uploads on a system that sanitizes/reviews the file before saving and therefore it would never allow a file in with a known hash flag.

The experience you're having is a reason to choose use a hosting provider that allows multiple user account per human/company. (Many providers have a very strict limit of one account per human/business and enforce sub-accounts.)

As you're seeing, an issue on one sub-account or site can sometimes bring down an entire account and all related sites.

Hope you are provided backups, they likely do have them, but you may need a lawyer to get more info and you'd need to get in touch before the typical backups get rolled over/deleted.