Hi Monkeyman,
If you can't see the hard drive listed when running:
sudo fdisk -l
I would start at the bottom which means going into the BIOS when the computer starts up.
IF you can't see it there then
- there is no use of looking for it in your OS.
- open the case and see whether it is spinning (or dead).
- IF it's dead/not spinning: check all the connections, swap cables with a hard drive that works.
Let us know where you're at. Then we can go from there.
In general (I know it unfortunately won't help you right now with that specific hard drive - BUT with the other 3. You might be aware of it anyway, too.)
* The 3 most important tasks for a computer user:
1. backup
2. backup
3. backup
* It sounds harsh but it's unfortunately true: If you don't run backups then you ultimately don't need the data as a hard drive is a consumption device, i.e. it will ultimately be "used up" at some stage and die. Always!
* Trying to recover data from a broken hard drive is possible in some cases BUT VERY expensive IF you need to get professionals involved. Far more expensive than plenty of backup tapes/disks.
All the best!