On another PC, download the
Windows 10 media creation tool and use it to make a win 10 installer on USB
change boot order so USB is first, hdd second
boot from installer
on screen after languages, choose repair this pc, not install.
choose troubleshoot
choose advanced
choose command prompt
type chkdsk C: /f and press enter - might take a while depending on drive size
other option is start up repair but it might just go back to spinning
boot from installer
on screen after languages, choose repair this pc, not install.
choose troubleshoot
choose advanced
choose start up repair - this will scan PC and maybe fix this - will ask for logon info
if neither help, is there anything on PC you cannot lose, try this: http://www.howtogeek.com/howto/windows-vista/use-ubuntu-live-cd-to-backup-files-from-your-dead-windows-computer/
I would try a reset next
boot from installer
on screen after languages, choose repair this pc, not install.
choose reset this PC
Two choices, save files and settings (which saves library folders, and logins) or wipe it all and install win 10 as new - I would keep files
Win 10 will restart and reinstall itself