I have an issue with Backup files

I've got an issue and don't know how to solve

1- I had a Hyper V installed on windows server 2012 r2
2- I've installed and configured file server and configured windows server
3- I've configured backups by windows server backup to take backup from the needed files on a VHD disk
4- Hyper V server went down and I couldn't find all the files and folders on the virtual disks after attaching the old virtual disks
5- I was able to attach the backup VHD partition to restore the backup as pint no# 3
6- after mounting the VHD the partition say it's unallocated and need to be formatted

So please could someone help me with this issue :(