I've recently heard from folks where the latest patch (KB2919355 or Windows 8.1 Update 1) bombed completely during installation, leaving their computer an unusable mess. It's rare, but is has happened. In one instance, after further investigation, it was identified that the core problem was OS files had become corrupted. The computer had not successfully installed updates for a few months with no indication they had failed. The update problem was also due to the corrupt OS files.
Microsoft support suggested running the System File Checker (SFC) utility, and of course, this was how the corrupted files were identified. However, when SFC finds damaged files it asks for physical media from which to locate and replace the problem files. Most people running Windows 8.1 upgraded from Windows 8 using the Windows Store, so there are no physical files from which SFC can pull from. So how does one get access to the file required to fix the operating system?
Microsoft offers the ability to install, download, or create media with an ISO file for Windows 8 or Windows 8.1 from a web page. It's not a secret page, but it's hard to find. To successfully use the download, all you need is your original, valid product key.
Here's the page: Upgrade Windows with only a product key