HELP! NAS suddenly reset!?

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I noticed my TS-439 Pro Turbo suddenly did not respond anymore. The display on the NAS said one of the fans had a failure. I powered off by holding down the power button and powered it on again. Now the setup wizard appears when browsing to the webinterface of the NAS......

HELP!!! What can I do to restore the NAS? How could this happen?

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ssh worked on the default port

ls only showed the .html file

I ran /etc/init.d/init_disk.sh

reboot

Same issue. Looks like the disks do not mount. Help?