volume inaccessible, how to rebuild after disk replacement?

[yoyos]

In storage manager volume section, the volume(RAID5) is now in "inaccesible" status, and in overview section it shows "error: system failure". Disk 6 in the volume is shown to be abnormal.

I replaced disk 6 with a blank disk of the same size (not new disk but is working well and filled with zero so should be the same as a new one), however nothing happenned. No RAID rebuild is triggerred. What should I do now?

[orion]

That should mean you got 2 or more disk failures on your RAID5 volume! It's a fatal error.
Are you sure only disk-6 failed? If yes, I'll suggest you to reboot NAS. If your volume is working in a degrade mode, that should be good. You can then replace with the new disk. NAS should re-sync your RAID5 volume automatically.

If it's not the case, I think you need to get helps from asustor support directly.