do you have a win 10 installer? On another PC, download it from
Here and create a USB or DVD.
Its better than a repair DVD as it can do just as much as the repair DVD and more.
try to boot from it, may need to change boot order in PC
if it loads installer, on second screen, after language choices, choose repair this PC, not install
choose troubleshoot
choose advanced
choose start up repair - this will scan pc and may fix start up - will ask for logon details
if this doesn't fix it, reload from installer and follow above steps till you reach are in advanced menu
choose start up options
choose one of the three safe modes
this will restart pc in safe mode, yuou don't have to do anything, sometimes just loading into safe mode is enough
restart pc and see if its fixed.
if it doesn't work, you may need a reset. If anything on PC you want to save, look at this: http://www.howtogeek.com/howto/windows-vista/use-ubuntu-live-cd-to-backup-files-from-your-dead-windows-computer/
another option is put hdd into another pc as a spare and copy files off it
now once pc is ready, boot from installer, select repair pc, and on first lvl, select reset this pc
can choose to keep files/settings or wipe it alll - either way you will need to reinstall all your programs again
once you choose, pc will restart amd reinstall win 10.