On what basis does the data center refuses to return your database? I understand that they can refuse to provide further services without refund of funds paid. I can even understand that they refuse to give the opportunity to transfer files to another data center, for fear that some of them would be illegal. But they can not give you information about the technical part of the service, that is, a database containing only the information about users and other technical information, but not the files? Display item in the contract that allows them to do so.
Database important for now. If not, we will receive a big refund request.