Can you send in the logs? If it's a multiple disk failure you can attempt recovery through Recovery service or by using third party apps like ReCLAIMe. If it's an issue with the System it would be best if you contact Support so they can assist you with getting your NAS working.
You may have to purchase a recovery contract or support contract in case your NAS is out of support or you need recovery services.
How much data (if any) can be recovered depends on what went wrong.
As @Marc_V says, if you want to recover it on your own, you'd need to put the disks into a Windows PC, and use RAID recovery software that supports BTRFS. The only package I am aware of that does is ReclaiMe ( https://www.reclaime.com/ ). You can install it before purchase, and see if it can recovery anything.
@Marc_V also offered to take a look at your log zip file first - which wouldn't cost anything, and would give you some idea of whether data recovery is likely to succeed. I suggest that you take advantage of that. Download the full log zip file from the NAS web ui, and then store that in cloud storage (google drive, dropbox, etc). Send Marc a private message using the envelope icon in the upper right of the forum page that includes a downloadable link to the zip. Don't post it publicly.
