Sounds like a bad disk, hence the failed RAID and inability to re-build it.
Switch the NAS off, take out the good disk and connect it to a PC directly if you can. Then use a linux file system driver from
http://www.fs-driver.org/ and mount the drive read-only. Then backup your data somewhere.
If you have a USB drive enclosure, or USB to SATA cable, it makes it even easier to do the above.
If you use Linux, you don't need any external software to read the disk.
You should keep backups of what's on your NAS.