The error message you have quoted indicates that you are trying to open a workstation database. You are are currently running the NV2 workstation which is a workstation database. Accessing a workstation database while running a workstation database will not display your application data regardless of what the file name has been renamed to.
From your backups, look for a file called database.nv2 or YourCompanyName.nv2 and copy the file to your hard drive and then open the application database.
There are three types of databases in NV2. The workstation database, which is usually called workstation.nv2 (this can be renamed), the application database (your books, which by default is named database.nv2 but can also be renamed), and the server database which by default is named server.nv2 (this is used if you are accessing the application database in multiuser).
Regards to all,