You'll need an Install Disk or Repair Disk. This means using another PC running Windows on the same architecture as yours (32/64bit) to download the Media version (ISO) to create an install disk,(Edit- must be running Vista or W7) or another machine running W8 to create a Repair disk. You'll be able to open the download link and use your Product Key to download the ISO http://go.microsoft.com/fwlink/?LinkID=262204
Then burn to a DVD or bootable USB drive. Boot from whichever and choose the Repair option. Here you can use various tools to repair your fault.
If you can use the Repair disk Option, on a W8 PC type 'Create' at the Start Screen and from Settings choose 'Create Recovery Drive'.