It's easy.. just download Microsoft's Windows 10 Media Creation Tool. With that you can create an ISO or bootable USB drive to install Windows.
https://www.microsoft.com/en-us/software-download/windows10
Before you do anything... I would first check and verify that your Microsoft account is still valid.. otherwise you might have the same problem after you reinstall. The easiest and fastest way of this this would be to either go to Bing or Outlook.com from any browser and log in using your Windows account. If you're able to do it then you're good to go. However, if you can't log on... I would try to some kind of password recovery FIRST... then do the reinstall.