Lost RAID - how do I recreate without losing the data on the disks

I have a NAS540 which started to beep constantly.
All the disks seemed to be working fine and there were no reports of degraded disks/RAID
So I re-flashed the firmware from USB, but the beeping remained.
I then removed each of the HDDs, and reinserted. This seemed to fix the beeping (eventually).
However, I have lost the disk group and RAID volumes as a result.
I suspect one of the HDD had lost its ID, corrupting the volume.

If there any way to recover the RAID volume?
If not, can anyone recommend how to recover the data on the HDDs?

Thanks

#NAS_May