Most likely file system on system volume, where configuration is kept, got corrupted. Make sure your RAID, if you use any, is ok and run file system repair tool from DSS console. If it doesnt help please open support ticket from user portal and attach logs from system.
Most likely you are running old version of DSS V6. In newer version it is possible to repair system volume even if you have no NAS volumes. Please update and then try to repair FS (download new ISO file from our web site and upload it in software updater field).
I downloaded dss6.0up95_b6156.oe_i.iso. But if I choose "S/W upgrade" in the webgui, it says: "it is not possible to upload an update file if the system volume does not exist".
Can I upgrade anyway? Using the gui? Is the config preserved then?
You can copy content of ISO file on USB stick and run /boot/bootinst.sh or /boot/bootinst.exe if on windows to make it bootable and boot DSS V6 from it to run file system repair. Later you may boot from old installation medium to save setup files to restore it on new version.