Modern NAS systems usually use Linux (ext4 or Btrfs) and manage disks with mdadm or LVM. When the NAS says the volume has failed (Crashed Volume), the data is usually there, but the logical mounting failed.
Recovery Strategy
Do not try to repair the volume from the NAS interface, as parity repair processes can be destructive if there are unstable disks. The correct way is to connect the drives to a PC with Linux (or use RAID recovery software on Windows like UFS Explorer), virtualize the array by detecting metadata, and copy the information to another safe location.
