79 — service error
Despite the wording, this is usually not a hardware failure. It generally means a corrupt print job or a firmware problem, so it's worth trying before you call anyone. Turn the printer off, unplug every cable including the data cable, and leave it for a minute before plugging it all back in. Then clear the print queue on every computer that sends to this printer, because one bad job sitting on a machine in another room will trigger it again the moment the printer comes back. Update the firmware if it keeps returning.
Related codes: 79