Marie-Noëlle,
Chances are that the disk has not been able to spin back up following the electrical brownout. Plug your ear on it, if it's not humming, it's dead. You can try unplugging it, shaking it, and turning back on the workstation, just to see if it helps, but that's long shot.
If you have another similar workstation with a DDS tape drive, you can replace the bad disk then clone the good workstation using make_tape_recovery.
In all other cases, if the data on the disk is important, it's probably recoverable but you'll have to hand it out to a specialized firm, one that can read HFS volumes. HP can probably help you finding someone who can do this. I have vague memories of cracking open a dead IDE drive a long time ago to help it respin one last time, and I was able to recover what was on it. But that's a stunt I wouldn't try with production data.
Bonne chance
