My Windows 10 was from a free upgrade of Windows 8.1.
I was getting back to my system, for continuing the work of the previous day, yesterday. The system was not switching on, and was stuck in Diagnosis menu, for many hours. I had a USB back of my upgraded Windows 10. I tried to boot/recover from the same, but it resulted in formatting my entire HDD and loosing all my recent data. It had successfully installed Windows 8.1, to the factory settings. It took the whole night and end after nearly 8 hours.
Now I would like to get back to Windows 10, as I was used to it since last 2 years or so. What to do?