Entire email accounts deactivated and deleted

Many years ago, our entire family had Xfinity internet service and had email accounts associated with that service.  The service was terrible and the internet just didn't work in our area, so we cancelled the service but we were instructed we could keep our email accounts.  When a family member moved to college a few years ago, they put the email accounts on her CURRENT internet service account.  We've had these email accounts for 15+ years.  Suddenly, 2 of those accounts can't even be logged into.  Another one will let me log in, but when I did all my past emails had been deleted.  All the folders (inbox, sent, deleted, folders)- all completely empty.  We've spent hours on the phone trying to remedy this, but the account representatives we've spoken with do not understand what has taken place.  They keep saying the only way they can help is if we provide the account number from our service 15 years ago.  I don't need to change anything on internet service I cancelled 15 years ago, I need the appropriate technical department to recover the email accounts.  We do not need them for primary use, but there is important information we would like to backup and are fine to deactive the accounts after that has taken place.  How do we find someone who can comprehend the issues that are taking place?  Do I need to file a formal complaint?  Start writing reviews and drawing attention to this on social media?  How do I reach a representative that can actually assist?  Thank you for your time.