Molly, are you sure you've fully powered off and it's not stuck in Hibernate? Press and hold the power button in for 30 seconds until you see all the power go off. Wait and then press it again to start it up and immediately begin pressing the BIOS Boot Menu key for your model shown in the table in steps above.
Do you see the BIOS splash screen (with your PC or BIOS logo on it?).
Can you enter BIOS/UEFI setup using the key designated for that here:
https://www.mydigitallife.net/comprehensive-lis.... If so reset it to default, check that UEFI is enabled, Secure and Fast boot are disabled until later, BIOS Boot Priority has Windows Boot Manager (for UEFI) or the hard drive set first to boot.
If you can't access the BIOS/UEFI Setup to do this, then try another of these methods to reset the CMOS:
https://www.maketecheasier.com/how-to-reset-cmo...
If none of these work, remove the hard drive and slave it to another PC to copy out any of your files then wipe the drive with Diskpart Clean Command: http://www.sevenforums.com/tutorials/52129-disk....
Power down to remove any other drive from the PC and follow the illustrated steps in this link which compile the best possible Clean Install of Windows which will stay that way as long as you stick with the tools and methods given, has zero reported problems, and is better than any amount of money could buy: http://answers.microsoft.com/en-us/windows/wiki...
Once WIndows is installed but before the screens for User participation, power down and move the hard drive back to the problem PC to see if it will start now, complete the install following the steps closely - driver steps are important enough they are printed in red.