Readynas 104 lost volumes

What you experienced is why Netgear (as well as StephenB​ and I) recommens a backup before expanding.  In fact, we recommend you always have a full backup.  The sync process puts a lot of stress on the remaining drives, so the failure possibility of of them is increased during that time.  And if one fails before the sync completes, as yours did, then your volume becomes dead.  I'm not saying this to chastise you, but as a warning to anyone else reading this thread.  Too many think RAID is a guarantee against data loss, which you have found the hard way not to be the case. 

That said, most of your volume is likely recoverable since the replaced drive was not due to failure.  The removed and re-inserted drive is "just" out of sync with the ones that remained, but that's usually fixable when there is only one drive failure.  So don't panic and do something that may make things worse.  Do you know if you made any changes to files while the unit was re-syncing?  Any file modified or added is probably not recoverable.  Once StephenB​ has your logs, he can likely guide you through a successful recovery.

Once recovery is complete, you need to think about backup.  With all ReadyNAS being EoL, you may want to think about buying another brand of NAS and relegating your 104 to backup duty.