It sounds as though the data didn't get backed up in the first place. To investigate further you can rename .mbz to .zip and then unzip the backup file. Poke around in the moodle.xml file and see whether you can find any evidence of recorded grades. Also check for uploaded files. You should be able to satisfy yourself that either the data is present in the course backup or it is not. If the data is present then the restore process is malfunctioning (unlikely), if not then read ahead ...
If none of this works I'd suggest asking the system manager (perhaps yourself) for a copy of the database dump made prior to the deletion of the vital course. Then in a sandbox system (I've used MAMP on a Mac for this purpose) create a temporary moodle using a copy of the production code and the database with the vital course. You may not need the moodledata at all. This will have the vital course, enrollments and grades (depending upon the date/time that the database dump was executed) and you can redo the course backup from here. I've used this technique in an emergency to restore a course I accidently deleted.
Hope this helps.