No bootable devices found. XPS 15 9560.

I have a feeling it might be the USB itself, am trying on another USB, and will update this post if anything happens.

Hello, I managed to get the USB working and have gone into trying to repair the PC (as opposed to reinstalling windows 10) by trying to get the bootloader. However it seems that I cannot access the hard drive and what seems to be of the hard drive is empty (as if it was formatted). Now I have no idea how this happened, but I've been trying to avoid installing windows 10 altogether and repair the boot. But I'm incredibly dumbfounded that the hardrive appears to be completely formatted.

I think I've had the same problem, or at least, a very similar problem. Mine happened after I updated the BIOS. I would like to second the OP's question about the meaning of "Primary Drive = {none}" line in the System Configuration - my system is also behaving in that manner. The long serial number on the NEXT line matches that of my SSD, so the SSD is definitely showing on that page. The reason I ask is that the Dell tech assumed my SSD was defective because of that line showing apparently no primary drive, and was all ready to replace my SSD. I didn't trust their diagnosis, and instead opted to try repairing the Windows installation, which worked, except that I had to re-install all my applications. (all data was retained, EXCEPT my music, which I found most odd)

I didn't need to use any Windows media for the Windows repair.

The system is now mostly working, except that very intermittently, it will BSOD with the error "UNEXPECTED STORE EXCEPTION", when I attempt to resume from sleep. Mind you, it hasn't happened for quite a while now - I've made a few minor settings changes, and I notice that there was a recent Intel update - fingers crossed.

So - does it matter that it says "Primary Drive = {none}"? How can it be that it thinks there is no "primary drive", yet the system is booting?

It's possible that in my case updating the BIOS changed the controller mode to RAID - I have read that this can prevent Windows from booting.  One other little thing - I noticed that Crystal DiskInfo was not identifying my SSD. I have now changed my controller mode to AHCI (using this procedure ), and Crystal DiskInfo is now identifying the SSD. However, it still says "Primary Drive = {none}" in the BIOS. Just perhaps, all I had to do was change the controller mode after the BIOS update, and all would have been well! 
EDIT: I see that changing the controller mode was already recommended to the OP, and it didn't fix the problem. 

 " Mind you, it hasn't happened for quite a while now - I've made a few minor settings changes, and I notice that there was a recent Intel update - fingers crossed "

:BigSmile:

I'm assuming the computer working normally and without any problems. 

So - does it matter that it says "Primary Drive = {none}"? How can it be that it thinks there is no "primary drive", yet the system is booting?

It appears your computer is using solid state drive as the boot drive and the hard drive as the storage drive. 

Deerdido

I'm assuming your computer is using the 500GB SSD as the boot drive and the hard drive as the storage drive unless it's the other way around.