This is unfortunate and unconscionable. Assuming you will not go for the lawyer route, I can only offer how I might write an appeal to them:
Hello,
I am writing to plead for the return of my account, or at least my data.
My Customer ID is K1027793822.
The background here is that on [date and time] a bad actor appeared to have gained control of my account. The email on the account was changed from [from] to [to]. I have this email as well as precious invoice emails that show I am the account holder.
I don’t know what actions they took that triggered the account suspension, and while I’d like to know everything the account did, my primary concern is retrieving my data.
Please let me know what you need from me to process this request.