if start up options isn't a choice, it gets harder to fix.
one thought is to run sick checker.
boot from installer, on 2nd screen after languages, choose repair this pc, not install
choose troubleshoot
choose advanced
choose command prompt
type chkdsk C: /f and press enter
this should check hard drive. restart PC and see if it helped
try system restore:
boot from installer, on 2nd screen after languages, choose repair this pc, not install
choose troubleshoot
choose advanced
choose system restore
pick a date from list, preferably a recent one
System will roll system settings back and maybe fix this
is there anything on C drive you want to rescue? try making this on another PC: http://www.howtogeek.com/howto/windows-vista/use-ubuntu-live-cd-to-backup-files-from-your-dead-windows-computer/
if you copy everything off C you want to save, I would try a reset
boot from installer, on 2nd screen after languages, choose repair this pc, not install
choose troubleshoot
Choose reset this PC
choose keep files/settings (Files = library folders. Library folders = Documents, pictures, movies, music (the default folders that come with windows). Settings = logins and desktop)
Other choice is Wipe it all which will reinstall win 10 as new
Which ever choice, PC will restart and reinstall win 10. You will have to install programs again. If you ahve more than 1 hdd, the other drive will be left alone by reset so all contents will still exist.