Asus computer relatively unused, windows 8 has been stuck on scanning on repairing. no matter what I do restart or leave it for a day it is still stuck.

The full screen is a loading screen with the caption:

"Scanning and repairing drive (\\?\Volume{f7a7bad6-c587-49d5-afc5-d9d7af628978}): 75% complete"

restarting it from here just gives me an auto repair caption than a black screen that I can run my cursor around on

what can I do?