It looks like the person's evaluation copy of Windows 8 Release Preview (and not Windows 8.1) has expired and that is why it is shutting down.. it expired January 15, 2013.
- The desktop background will turn black and your current wallpaper is removed. While you can still change it back, it will be removed again in regular intervals.
- A permanent notification is displayed on the desktop stating that the copy of the operating system is not genuine
- The PC will shut down every hour automatically so that you may lose unsaved work in the process.
Upgrading from Windows 8 Consumer Preview to Windows 8.1 Preview is definitely not supported. Even if you managed to download the upgrade fast enough, this will almost definitely require a clean install.