NAS Device Failure
When a NAS stops responding, it's easy to assume the drives inside have failed together — but very often the actual problem is the small computer running the NAS's own operating system, not the multiple physical disks holding the data.
What's actually happening
A NAS is effectively a dedicated small server: it runs its own operating system — Synology's DSM, QNAP's QTS, and similar — which manages the RAID or hybrid-RAID layer, the file system sitting on top of it (often a proprietary or lightly modified variant of ext4, Btrfs, or similar), and its own metadata describing how everything fits together. If that management layer corrupts, the NAS's motherboard fails, or a firmware update goes wrong, the whole unit can stop responding even though every drive inside is completely healthy.
How recoverable this really is
The odds tend to be good here, because the drives themselves, and the standard file system sitting on top of the RAID layer, are frequently unaffected. The complication is that many NAS vendors add proprietary elements — Synology's SHR (Synology Hybrid RAID) is probably the best-known example — that a generic RAID recovery approach won't automatically make sense of without vendor-specific knowledge of how that particular layout works.
The general approach
Recovery generally pulls the drives from the dead unit and connects them directly to a separate system, reconstructing the array and volume from the drives' own data without depending on the original NAS hardware or software working at all. Vendor-specific layout knowledge, or in some cases a replacement unit of the same model once the drives themselves are confirmed healthy, fills in whatever proprietary logic is needed.
Don't factory-reset the NAS or reinstall its operating system hoping to "fix" it. On most platforms, that's a fast way to wipe the very management data the volume depends on — even though it does nothing to the actual file data sitting further down on the disks.
Practical, step-by-step guides for specific recovery software will be added here as separate linked articles.
This page describes the general situation, not a guaranteed fix for your specific case. If you'd rather have someone experienced take a look before you try anything, see what a hands-on evaluation looks like.