If I'm not mistaken, the default settings in OMV Backup are to use 'dd' to save a (compressed) image of the entire system partition, although fsarchiver is another option.
So chances are that the OP can restore the backup, maybe through Clonezilla (?) or, if all else fails, from a Debian live session booted from a USB stick.
The problem is I don't remember which method I used in the omv-backup. But if I remember correctly it is not a single image, instead it is multiple files and directories. So I guess it is fsarchiver or rsync?
Can I convert this into a compressed image to use with Clonezilla when restoring?