A NAS is a small computer with disks attached, and a failure anywhere between the network and the file system produces the same experience at your desk. Working out how far along that chain things still work takes a few minutes and changes everything that follows.
Network, then share, then volume, then array — in that order, because each is cheaper and safer to test than the one beneath it. Going straight to the disks is how a permissions problem becomes a recovery job.
You cannot reach it on the network. IP change, DHCP, switch or cable. The unit and the data are both fine. Connect a laptop directly to it before assuming anything else.
The unit will not power on. Usually the external power supply rather than the disks. Consumer NAS bricks fail regularly and the disks behind them are untouched.
The volume is degraded but working. A disk has dropped and redundancy is holding. Everything still works and the margin is gone. Copy your data off now, before anything else.
The volume has crashed or unmounted. The genuine data problem. Recoverable in most cases, and considerably more so if no rebuild has been attempted since it happened.
Whatever the vendor interface looks like, nearly every consumer and small-business NAS runs ordinary Linux software RAID underneath — mdadm arrays, usually with LVM, and either ext4 or Btrfs above. The disks are not written in a format only that box can read.
So if the electronics have died, the disks can be read on other hardware entirely and the array reassembled from their superblocks. You do not need a replacement unit of the same model, and you certainly should not send them to the manufacturer.
What adds steps is the layers above. Synology Hybrid RAID partitions across mismatched disk sizes. QNAP uses LVM thin provisioning, adding a mapping layer that must be rebuilt before the file system is reachable. Both are well understood.
Usually two disks out of a RAID 5, or file system damage on top of a healthy array. Do not repair, do not rebuild. Power down, label the bays and send every disk.
Almost always recoverable. Standard mdadm arrays can be read on other hardware and reassembled. A replacement unit is not required.
Yes, labelled by bay. The array reconstructs from the full set and bay order is part of the structure.
Frequently a thin-provisioning metadata fault rather than disk failure. The map between logical volume and physical blocks has been damaged. Do not let the unit attempt a repair.
No, and this misunderstanding is behind most NAS jobs we see. RAID protects against a disk failing. It does nothing about deletion, ransomware, fire or theft.
From £500 +VAT for multi-bay units, £300 +VAT for single-bay, fixed after the free diagnostic.