Try pressing F8 repeteadely (about once every 1 to 1.5 second)... and from the Safe Mode options list, select:
a) The Startup Repair; It may be available depending if the tools were installed on the hard disk...
b) Start in Safe Mode from which you can run System Restore.
If the Startup Repair tools were not installed on your hard disk, you will need a Windows installation disk.. If you don't have one, you may be able to download a Windows 8 ISO to create a installation disk from which you can run the Startup Repair to fix the bootloader. To read this information in full detail, click the link and next click on "Repair your computer"
The suggested download is a Windows 8 Enterprise evaluation ISO that should repair the Startup sector no matter if it's a different Windows 8 version to the one you have installed. You'll need to register before you can view the download link but you know it's a safe download and there is no commitment to purchase the download.
http://www.microsoft.com/en-us/evalcenter/evaluate-windows-8-1-enterprise
How to Boot into Safe Mode (F8) in Windows 8 & 8.1
http://www.mydigitallife.info/boot-safe-mode-f8-windows-8-8-1/