Sorry to hear that. That is the unfortunate problem with all such devices. The controller itself failing can destroy data on all the disks in the case of Raid5. RAID5 is not suited to home use as a way of backing up data. RAID5 is ideally suited to business use where small amounts of data are required quickly ie databases.
The best way and most cost effective is to run a NAS with justa bunch of disks. And then keep backups of these on USB HDs off site somewhere.
With regard to your current issue... the data maybe ok. Speak to Buffalo.
Even the best RAID controllers fail.