It's important to do the RAID health check and make sure that the RAID is 100% healthy before running the file system repair. You shouldn’t run it when the RAID is rebuilding because there is a high risk of data loose.
If the file system is corrupted on the system volume or on the NAS logical volume, it won't be mounted. That's why the file system needs to be repaired. If the file system is badly destroyed there is a risk that data can be lost.
But in your situation most probably the DOM became corrupted and it needs to be replaced. It sometimes happens especially with the old DOMs.