Does anybody knows what will happen when DNS 320 got Crushed, is it hard to recover and what shall i do...
If the DNS-320 is physically damaged and the HDDs are still intact, you can insert the HDDs into a new DNS-320 and the data should be accessible. As an alternative, the DNS-320 formats HDDs using the Linux file system, so you can mount the HDDs in a Linux PC (or Windows PC with an app that reads the Linux file system) to extract the data.
If the DNS-320 crashed, you can reset the unit to the factory defaults using the button on the rear of the device. If this doesn't work there are other options available.