on another pc, download this and run it to create a Win 10 installer on USB
Win 10 media creation tool
in the BIOS, change boot order so USB is first, hdd second
boot from installer
on screen after language choices, choose repair this pc, not install
choose troubleshoot
choose advanced
choose start up repair - this will scan pc and maybe fix this -0 will ask for logon info
if that din't help
reboot installer, choose repair this pc
choose troubleshoot
choose advanced
choose start up options
choose one of the three safe modes available
pc will restart in safe mode - if this works you don't need to do anything, just restart pc. Sometimes just going into safe mode can fix this
'
See how that goes, if it still doesn't help, you can try this to save anything off PC you want to keep: http://www.howtogeek.com/howto/windows-vista/use-ubuntu-live-cd-to-backup-files-from-your-dead-windows-computer/
Try a reset
boot installer, choose repair pc
choose reset this pc, you can keep files and settings or wipe it all - either way need to reinstall all programs
once you choose pc will restart and reinstall win 10
if that fails to work, you might need to try this:
http://www.thewindowsclub.com/the-drive-where-windows-is-installed-is-locked
or do a fresh install following this:
http://www.tenforums.com/tutorials/1950-windows-10-clean-install.html
Good Luck