Here is a basic restore procedure using fsarchiver for the same hard drive but a change went bad:

Boot systemrescuecd - if you installed the iso from omv-extras, it will be in your grub menu.
Mount your data drive - example:

mount /dev/sde1 /mnt/backup

Figure out where your backup file is. fsarchiver uses the .fsa extension - example:

/mnt/backup/omvbackup/backup-omv-07-May-2018_10-36-31.fsa

Figure out what your OS drive root partiton is. Don't get this wrong! - example: /dev/sda1
Restore the files - example

fsarchiver restfs /mnt/backup/omvbackup/backup-omv-07-May-2018_10-36-31.fsa dest=/dev/sda1
reboot

Here is a basic restore procedure using dd for the same hard drive but a change went bad:

Boot systemrescuecd - if you installed the iso from omv-extras, it will be in your grub menu.
Mount your data drive - example:

mount /dev/sde1 /mnt/backup

Figure out where your backup file is. dd uses the .dd.gz extension - example: /mnt/backup/omvbackup/backup-omv-07-May-2018_10-36-31.dd.gz
Figure out what your OS drive root partiton is. Don't get this wrong! - example: /dev/sda1
Restore the files - example

gunzip -c /mnt/backup/omvbackup/backup-omv-07-May-2018_10-36-31.dd.gz | dd of=/dev/sda1 bs=1M status=progress
reboot