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, pool or hybrid-storage layer, the file system sitting on top of it (commonly standard Linux technologies such as ext4 or Btrfs, sometimes combined with vendor-specific management metadata), and the services that expose the storage over the network. 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
Recoverability depends on whether the failure is limited to the NAS appliance or also involves the disks, array metadata, pool or file system. Many NAS layouts are built from recognizable Linux storage technologies, but vendor-specific assembly rules, metadata and features such as Synology SHR can still matter. A dead enclosure with healthy members is therefore very different from a degraded pool with unreadable disks.
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, initialize, recreate the storage pool, or reinstall the NAS operating system as an experiment when the data is the priority. The exact effect varies by vendor and model, but these actions can rewrite system, array or volume metadata and make later reconstruction harder.
If the NAS storage pool itself has lost redundancy or members, also read RAID Array Failure and Failed or Interrupted RAID Rebuild. If any of the pulled drives are also struggling to read reliably, imaging them first with purpose-built imaging hardware is worth doing before attempting reconstruction.
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.