For a degraded or crashed volume after a power outage, the knowledge base says the usual cause is drive errors or file system errors.
What you can do, depending on the situation:
If the volume is degraded
If the volume is read-only
If the volume is crashed
For DSM 7.1 to DSM 7.4 SHA troubleshooting, the guide says:
1. On the active server, go to Storage Manager to find out which storage pool the crashed volume belongs to.
2. Go to Storage Manager > HDD/SSD to find out which drive crashed and replace it with a healthy one.
3. Go to Storage Manager > Storage, select the crashed storage pool, and click Repair.
Also, the KB recommends:
- use only drives in the Synology Products Compatibility List
- use a UPS to reduce the risk of file system errors after improper shutdowns
If you want, I can also summarize the safest recovery path specifically for DSM 7.0, DSM 7.1–7.4, or SHA.
Sources used: [1], [8], [9], [12]