Knowing little about Windows 10 it's interesting, though irrelevant, where various nonworking features of an upgrade from 64-bit 8.1 Pro to 64-bit 10 Pro came. To the OP and myself (and apparently many many more - see this forum and forums around the 'Net) what REALLY matters is a relatively painless upgrade resulted in a useless, really broken OS.
I performed two upgrades today - the first, as noted, broke amazingly hard, the 2nd apparently is working fine (I don't personally know that however I sicced my daughter on the upgraded 64-bit 8.1. Home laptop, figuring she'd be using most features by the time she hits the sack sometime around 3AM...repeatedly asking her "How's it going?" with "No problems yet!" as the responses is reassuring).
Besides the "level" of the 8.1 to be upgraded (Pro/Home) there is one other, possibly critical, difference in the two upgrades - for the 8.1 Home laptop I simply waited until Microsoft said "Yeah, go ahead, you're cool".
But with the 8.1 Pro desktop I modified a registry key that says essentially the same thing - info from a weblog by Dave Gardiner
weblog/ - that actually got around "The Wait" but, instead of using what was already downloaded onto my machine, downloaded a NEW copy that was used for the upgrade. Though I believe this paragraph's contents may not be relevant, either, there IS the possibility that something else with the upgrade procedure doesn't really like the customer getting around a locked door; for example, it is possible that uSoft is holding off enabling upgrades for certain machines/setups until 3rd party developers get their acts together and upgrade their drivers, etc - forcing the upgrade as Mr. Gardiner successfully did and I (and the OP?) unsuccessfully did might be a side-effect.
In any case, I'm sitting on my hands for now - my Clonezilla recovery of my recently backed up C drive got me back to July 30th's 8.1 where it'll stay until uSoft says "Yeah, go ahead, you're cool" to this machine.
EDIT - I just saw a post/article that mentioned that "Core 2 quad" CPUs may be problematic with Windows 10 - that is the CPU in the older desktop running Windows 8.1 Pro where the upgrade broke. Note that uSoft "precleared" our PCs prior to any attempts at upgrades.