Ugh, I know the feeling. I hate it too, but sometimes we have to reinstall Windows for one reason or another. I'm betting you lost your Windows key and the machine no longer has a Windows key sticker on the bottom. (assuming it's a laptop)
No worries, let's get you that Windows key. Its quick and simple.
- PressWIN + R. This will open the Run dialog.
- Typecmd.exeand press Enter/Return.
- Now typewmic path softwareLicensingService get OA3xOriginalProductKeyand press Enter/Return.