I'm assuming the H: drive is a CIFS share from NetApp. Did anything change on the NetApp side? Is it just one user, or multiple users?
If you can't find the file(s) in the snapshots, then the report is highly suspect. Depending on your snap schedule, it's conceivable that a file was created, then the upgrade happened, the file got lost (HIGHLY doubtful), then a snap happened. In this scenario the snap would have missed the file.
In my experience files don't just "vanish." 99% of these accusations turn out to be user error. Get the details from the user on when the file was created, verify the path, name, etc., and double check snapshots. If the only thing that happened in the environment was a Windows upgrade, then the fault (if there is any) lies there.