Curious. I think we need a little more information to hazard a guess at what's going on here. What exactly is the machine? A commercial build - in which case we need the exact make and model? Or a custom build - in which case we need the motherboard make and model, the HDD makes, models and types? What operating system are you running? What power supply (PSU) make, model and capacity are you using?
From what you've said: All the "Reboot and select proper boot device" message is telling you is that the BIOS cannot find a disk with a viable operating system on it. That would be so if, for some reason it can't "see" the system drive at all or if it can see the device but cannot identify it because the device parameters in the BIOS are corrupt. This could happen (I had one once) if for some reason, the missing HDD needed a manual setup, rather than autodetection and your hard power down had corrupted the settimgs. Why you would be using such a disk is unknown.
Except that brings us to the other oddity that Bob_B spotted - this so called dual cable. The GTX 970 has a PCIe connection slot and the processor is a Core i5, which roughly dates the machine to the time that SATA HDDs had replaced PATA drives in consumer PCs. The 1 TB size also points to this being a SATA drive, I don't recall PATA drives of that capacity. But while it was common to daisy chain PATA drives (or even SCSI drives, though these were rarely found in consumer class PCs), SATA drives are not. So what exactly is this dual cable? Is it two separate cables out of the motherboard but with the plugs moulded together because the headers were tight on space (I've seen this too)? As James notes, SATA power cables are often daisy chained but data cables are not.
Reading your post and follow up, it isn't obvious whether you've actually looked into the BIOS or just pulled up the boot menu? You do need to look into the BIOS and see what is in it for the HDDs. If there is, indeed, only one, then as Bob P. suggests, it's most likely a connection problem or a hardware failure in the SATA controller, data cable from it to the HDD, the power cable from the PSU to the HDD or the HDD itself. Unplug and replug all the connectors and see if that helps. If you just touch the outer case of the HDD, you should be able to feel or hear whether it's spinning. If not and you have a free power cable try that and if it still isn't spinning, you likely need a new HDD and reload it from your system image and the emergency repair disk you made. You did make one, didn't you? It's also worth replacing the SATA data cable and if you have a spare SATA connector on the motherboard, try that one.
If the BIOS is showimg both disks but one has corrupt or missing settings, you will need to reset them to HDD manufacturer's spec. Or, if it's set to manual, you could try setting it to autodetect and reboot and see what it comes up with.
NOTE!!! This is all "Best Guess" based on insufficient information. Your call whether you use all, any or none of it! You may think it better to take the machine to a repair shop for diagnosis.
You can also try James's other suggestion of a live Linux CD to boot off and take a look round the machine, which may give more clues. Personally, I use Knoppix because it will mount anything it can identify, when Windows may not but any Live Linux CD/DVD will do. Or try your Windows installtion disk in repair mode. If it can find a copy of Windows, it will try to repair it.