From command prompt type"
chkdsk /f
You'll get a message that the volume is mounted and will ask you if you want to run chkdsk at next startup. Say yes to this and reboot.
Chkdsk will examine the file structure of the volume and with the "/f" flag, it will attempt to fix issues.
Hope this helps!
then if it doesn't stick your 8.1 media in and either do a repair, if it doesn't work you may need a new hard drive if it gives any HDD errors