LukeFatwalker :
Sounds like your Master Boot Record (MBR) is either damaged or corrupted
Your two solutions are that you try resetting the MBR, if that doesn't work you can try a clean install of the OS
Guides:
http://www.thewindowsclub.com/repair-master-boot-record...
https://www.howtogeek.com/224342/how-to-clean-install-w...
Once you are back up and running, immieidately run a backup of the PC, a free tool like Rollback Rx or Macrium Reflect will work.
Thanks for replying! I tried what you said and it failed the first boot and the second one (though the 2nd one was different). On the third try, it actually booted. I'll wait and see if my computer reboots by itself again after booting propely (which is what tends to happen.)
Regarding a reinstall of Windows, I've already tried that unsucessefully.