If you've just upgraded your Windows 8 (non RT) device to Windows 8.1 and suddenly upon reboot you're facing a quietly humming computer with a black screen of nothingness....don't panic. You've probably just got Master Boot Record issues. This can be a fairly easy fix, particularly if you've already created a Windows 8.1 Recovery disk. If not, you'll need to use your bootable install media for Windows 8.1 or find a working Windows 8.1 device and create a recovery disk.
We'll assume that you don't have your bootable install media, and you've found a working Windows 8.1 device. On the working device, open the Charms and select Search.
In the Search charm type 'Create a Recovery Drive'
You'll get a prompt to allow the Recovery Media Creator Wizard to run, make sure you allow it. Insert a writeable USB Key (**warning** all content on this key will be deleted).
In the Recovery Media Wizard you'll specify this usb key as your Recovery drive.
The Recovery Drive Creator will wipe the usb key, and create a bootable partition with the Recovery tools installed. Once the wizard is complete, remove the usb key, and boot your impaired Windows 8.1 via the new usb Recovery Drive.
You may need to hit ESC or F9 during the reboot to select your boot drive. Here I hit ESC and selected the USB key I created during the Recovery Drive Creator.
From the Recovery Disk Menu - select Troubleshoot.
From the Troubleshoot menu - select Advanced options.
From the Advanced options menu select - Command Prompt.
In the command prompt - type 'bootrec /fixmbr'
Hit enter. Quickly you should see a confirmation like above. Type exit and hit enter.
You have two options back at this screen...you can continue to Windows 8.1 or (I recommend) Turn off your PC and restart it.
If you haven't already, remove the usb key, and start your device.
You should be back up and booting into Windows 8.1.