On another PC, download the
Windows 10 media creation tool and use it to make a win 10 installer on USB << if you don't have an installer now
change boot order so USB is first, hdd second
safe mode: boot from installer, on 2nd screen after languages, choose repair this pc, not install
choose troubleshoot
choose advanced
choose start up options
hit the restart button
choose a safe mode (it doesn't matter which) by using number associated with it.
PC will restart and load safe mode
system restore
boot from installer, on 2nd screen after languages, choose repair this pc, not install
choose troubleshoot
choose advanced
choose system restore
if you already tried that, you might need a reset
try this to copy files off C you want to save: http://www.howtogeek.com/howto/windows-vista/use-ubuntu-live-cd-to-backup-files-from-your-dead-windows-computer/
boot from installer, on 2nd screen after languages, choose repair this pc, not install
choose troubleshoot
choose reset this PC
can choose to save files/settings - files include library folders like downloads, pictures, music, etc. Other choice is wipe it all - this only effects C drive, no other drives
either choice, PC will restart and reinstall win 10