RN212 all files gone except iCloud

On the problem itself - though it's too early to say what the chance of getting the data back is, it doesn't sound good.  Anything you write to the NAS will lower the odds.  So I would either shut it down, or reboot it in read-only mode.

It sounds you are saying that the NAS suddenly has a healthy but empty volume (apart from that one picture folder)  I am wondering

• If you had other shares (not the default ones) before that have now disappeared.
• if the failure might have given you a completely empty volume, with the iClouds photos resynced by ReadyCloud from your phone after the failure occured.

On the logs: One option is to contact the mods ( JohnCM_S and Marc_V ) via PM (private message), and ask them to analyze the logs.  Send them a link to this thread and a download link (google drive, icloud, dropbox, etc) link to the log zip file.  Don't post the log zip here.

When I'm looking at them, I always look in disk_info.log at the disk health, and mdstat.log to see what disks are currently in the volume RAID array (md12x RAID groups are the ones in the data volume).  I generally also check the bottom of volume.log, as that will tell me the OS partition fullness, and the maintenance bit at the bottom can also be useful.  

Readynasd.log and status.log will give you a high-level view of what the ReadyNAS application was doing, and that might give you some clues on what else went on around the time you had to reset your ReadyCloud password.

You can delve deeper by looking in system.log, kernel.log and perhaps dmesg.log though there is a lot in there.