Studio XPS 7100, will not boot (from HDD or repair disk)

XPS 7100 (bios: A06, 09/10/2010) will not boot.

Far from the first time this has happened with this computer. Probably a few dozen times over the years, to the extent that I have several repair disks floating around. And thankfully a second computer of exactly the same type - which has not had these issues - so I can always make a new repair disk. Have long since gotten to the point that I rarely turn this computer off because it is prone to these issues when restarting.

This particular time:

Computer had been on for some time. I look over and it had the standard No Boot Device Found screen (Sata1 & Sata 2 installed). Not sure why; we didn't have a power surge or anything. At any rate. It is set to boot from DVD 1st and HD 2nd. Have tried variations of: put in repair disk, make a new repair disk and put that in (in case there was issue with previous repair disk which is about 1 month old, ie we had similar issues a month ago), restore BIOS defaults then change to look for DVD 1st and HD 2nd, tried restarting a number of times, etc.

Did the diagnostic, everything was fine (including the take-forever memory tests) except Hard Drive not found (DST short test; error code 2000-0141). I want to say that I've seen hard drive not found before but was able to boot from the repair disk and the HD was eventually found.

At any rate, this bit about not being able to boot from the repair disk is a new one.

Any other suggestions?