Dell OptiPlex GX260 Series running Lubuntu.  Could have been a power surge caused a reboot, but boot won't complete.  Drive unrecognized.  Tried running diagnostics and everything passed.  Tried setting caps lock, NUM lock, scroll lock, then Alt E, Alt F, Alt B; now Drive 0 is set to OFF.  Tried F2 to reset Primary Drive 0 to Auto, but still black screen only after reboot.  Tried F2 again and noticed that drive 0 is back to OFF.  I checked forums (older ones, too), as I figure someone has surely encountered this before, but I didn't see anything helpful.  Anyone know a solution to this?If you try to install Ubuntu to the drive with a clean install the drive shuts off? If you havent tried a clean install you can try that to see if some sort of sleep MODE problem is causing this.

If during a clean install the hard drive goes out to lunch then I would replace the hard drive with a healthy one. If problem continues with a DIFFERENT healthy drive, then I would swap out the power supply with a known good supply. I have had power supplies cause hard drives to disappear or act up before yet the voltages are strong enough for the system to continue running but with a drive failure. I swapped out the HDD and the problem remained, my 12VDC was weak and so I swapped the power supply with a healthy one and that fixed the issue.At some POINT a new hard drive may be the only solution. Few hard drives now have a  five year warranty.  The mean time to fatal failure is still high for the group, but when a single  drive FAILS the statistic does not help.

Unless you have a hard drive service station in you home...
Buy a new drive.