Preserve the array as you found it
Do not discard the drive that appears to have failed. Its metadata or readable regions may still be needed to reconstruct the array. Label every drive with its bay number, photograph the layout and record any warning messages before powering the system down.
Avoid rebuild, initialise, repair or format commands until the failure has been assessed. A rebuild writes across the array and can turn a recoverable configuration problem into overwritten data.
Ask before sending the chassis
Some cases only need the member drives and a clear slot map. Others depend on a proprietary enclosure, controller, cache or configuration. Send photographs of the system, model, drive labels and reported RAID level first. We will tell you whether the full NAS or server is useful.