This happened twice in the last few days. On two different computers. To two different people.
Their computers had been upgraded to Windows 10 from Windows 7. Both computers spontaneously reverted back to Windows 7. But here's the worst part.
The only thing remaining from the prior installation was the user name and password. All settings, installed programs, documents, and printers were removed. It was as if there was a clean install of Windows 7. Everything wiped out.
The first instance, I thought it might be PEBCAK or 1D01T error. The second instance, there's something wrong.
Both victims are men in their 80's.
The first one came back from vacation to find his system unusable. Everything gone. Or so he says. I have no reason to doubt him.
The second one, took a dinner break. When he returned to his computer, he tried to resume what he was doing, and something went wrong. He and his wife tried to recover what they were doing. They are not that sophisticated that they should be able to get themselves into deep trouble on a computer. And they are normally careful. But the end result was that the whole system got wiped out and they are back at a vanilla Windows 7 system.
Of course, Microsoft support has never heard of this happening before (twice in two days so I don't believe them).
This is one of the dangers that can happen when the dominant OS manufacturer decides it has license to change customers devices at will after purchase.
On both of these clients, I really should have set them up with Chromebooks. I knew that M$ was just going to be continual problems.