Most likely it means the hard disk has failed. Sky's advice is to contact them: https://www.sky.com/help/articles/system-fault-06-0-osm
They'll undoubtedly urge you to upgrade as a solution. If you don't want to upgrade, you could look for a used Sky+HD box on ebay etc. Choose only a DRX890 or DRX895 model as they're the most recent, albeit still 8-9 years old, and expect to pay around £30.
Edited to add: you could try a System Reset, which might get the box working temporarily. You'll still lose all your recordings, however. The Reset option is in the Installer Menu: Services > 0,0,1 >Select on your remote.
