If you do, you should be able to roll back to the most recent snapshot. The process for that is to go to the settings for the share in the admin web UI, and then select "recover". That would bring up a snapshot list - you then pick the newest, and click on restore.
If not (likely the case if you aren't seeing recover enabled), you should immediately boot the NAS in read-only mode, as any writes to the array risks overwriting the data you want to recover.
Then you'll need to either attempt to recover files yourself using btrfs tools, or (likely better) use a data recovery service or data recovery software. You can potentially connect one of the disks to a Windows PC (usb or sata) and try ReclaiMe. Not you don't need to purchase until you try to actually recover something.
I also suggest putting a backup plan in place for your NAS. As should now be clear, RAID isn't enough to keep data safe.
