Sorry, I realize it's not quite that simple. Hopefully you have better luck with gpart. I can't fathom how it could just be destroyed like that. Have you looked at the SMART report on the HDD to verify that it's not dying?
Don't worry about it. I have an image of the disk from when I first installed Windows 10, and the only thing I risk losing is a few save-games, though it looks like I can recover those with TestDisk.
But yeah, it's rather strange. Checking the SMART report is a good idea, thanks, but smartcontrol reports that the overall health status is "PASSED", there doesn't appear to be any errors in the logs, nor any failures of attributes. It's also not a very old disk, less than a year old at this point.
EDIT: Well, gpart(ed) had no clue, but I was able to partially recreate the partition table with TestDisk. Not enough to give me a functional OS, but enough that it looks like I can easily pillage the stuff that I want to save. Plus, I might be able to avoid having to redownload every Steam game.