A NAS is a small Linux computer with disks in it, and the message you get is identical whether the network dropped, a share permission changed, the volume unmounted, or the array collapsed. Those four need very different responses.
Network, then share, then volume, then array. Each layer is cheaper and safer to check than the one below it, and jumping straight to the array is how people damage a NAS that only had a permissions problem.
Start with the boring possibilities, because they are common and cost nothing. Can you reach the NAS’s admin interface in a browser by IP address? If not, the unit may simply have taken a different address after a router restart, or the network port or cable has failed. If the admin page loads but the share does not appear, the problem is at the share layer — permissions, a disabled SMB version, or a user account that no longer maps. Windows dropping SMBv1 orphaned a great many older NAS units, and that presents exactly as “not accessible”.
Neither of these is a data problem, and neither requires anything to be dismantled.
If the admin interface loads but reports the volume as unmounted, degraded, crashed or read-only, you have moved from a network problem to a storage one. Read what it actually says before acting. “Degraded” means a member has dropped but the volume is still readable — copy your data off now, while you still can, before attempting any repair. “Crashed” or “inactive” means the array can no longer be assembled, and that is the point to stop.
A thin-provisioned volume that has filled its pool will also stop accepting writes and can appear inaccessible while being perfectly intact underneath.
At this layer the disks are removed, labelled by bay and imaged read-only, and the array is reconstructed from the images rather than by the NAS. Most units — Synology, QNAP, Buffalo, Netgear, Iomega — use standard Linux software RAID with a pool layer above it, so the reconstruction is well understood provided the members can be read. Drobo is the exception, using a proprietary layout that has to be derived rather than assumed.
What decides the outcome is what happened before it arrived. A NAS powered down promptly and sent with its bay order intact is a good job. One that has been left resyncing onto a failing member for three days is a much worse one.
Either works. If you remove the disks, label each with its bay number first — that order is central to the reconstruction. Tell us the model, the RAID level if you know it, and what has already been attempted, particularly any rebuild. NAS, RAID and server recovery is from £500 +VAT after a free 48-hour diagnostic and a written quote, with a 50% deposit where drive-level physical work is needed and the balance due only on success.
At the top. Check you can reach the admin interface by IP; if not, it may just have changed address or lost its network link. If the interface loads but the share does not appear, it is a permissions or SMB problem. Only if the interface reports the volume as degraded or crashed are you dealing with a storage fault.
Copy your data off immediately, before attempting any repair. Degraded means a member has dropped but the volume is still readable, and that window is the cheapest recovery available. Don't start a rebuild until the data is safe somewhere else.
Often because Windows disabled SMBv1, which many older NAS units still rely on. The NAS is entirely healthy; the protocol both ends agreed on has gone away. That presents identically to a failure but is a configuration issue, not a data one.
Either is fine. If you send disks alone, label each with its bay number first, because bay order matters for reconstruction. Don't reinsert a disk the unit ejected and don't let a rebuild run before it comes in.
NAS, RAID and server recovery starts from £500 plus VAT, fixed in a written quote after a free 48-hour diagnostic. Where a member needs physical drive-level work there is a 50% deposit, with the balance payable only on success.