The same happened to my husbands computer here at home. When he got home from work his computer was almost done up dating to Window 10.
He was furious!!!
When it completed the up date he said he wanted his older Windows....It stated it would take some time to get the old information back. He said he wanted his other Windows. It took about 15 minutes but it did return back...
I was pretty furious too, and it cost me about $2000 in easy missed stock trades yesterday which was an absolutely glorious day for trading.
I shut mine off while it was updating, and it was undone. At my job we use programs that are not yet compatible with Windows 10; how can they just do it without permission? That was the third time I stopped it by turning off the PC; I no longer leave it unattended, and shut down completely.