If you are past free update period and never made a working back up of the SD card from  your PC with a card reader and saved the older one  while you were still within the free update period, I think you are now screwed.
  If you have the DS708 PC suite you previously used and a new blank 16 GB SD card. Install the New SD card in the DS 708, then turn it on , shut it off , remove the SD card. Open the DS708 PC suite, put the SD card in the card reader and put it  in your PC and see if it says it has available updates.
  Otherwise you need to call support with the unit's serial number and register password to see if there is some way you can get it going again.

  redundant post, deleted , please disregard