Well if you don't have a repair disk, next best thing is to try and completely re-install your OS. If you have any external hard drives or UBS Sticks with at least 4Gb of storage. Here is what you need to do to make the thing bootable.
1. Download The ISO file (Trust me this version of windows is crucial to activating Windows)
[Windows 8.1 pro 64-bit]: http://getintopc.com/softwares/operating-systems/windows-8-1-download-iso-32-64-bit-official/
2. You will need Winrar to extract the ISO
Link:
http://www.rarlab.com/download.htm
3. Once that is done, Extract the ISO file to where you can easily find it (Like the Desktop)
4. Download the Windows 7 USB/DVD Download Tool (I know it says Windows 7, don't worry it supports any windows OS XP and up)
Link:
http://download.cnet.com/Windows-7-USB-DVD-Download-Tool/3000-18513_4-10972600.html
5. Plug in your USB or External Hard drive into your PC
6. Format the USB or External Hard drive to make sure is completely clean
7. Run the Windows 7 USB/DVD Download Tool and choose the ISO you downloaded
8. Click next and select the type of storage device you plugged in (Select USB if you plugged in a USB stick, and choose DVD if it's an External Hard drive)
9. Then Select the drive via the drop down menu (If you don't see the device click refresh, and if that doesn't work unplug it and plug it in again and click refresh)
10. Click begin copying and then wait for it to finish
11. When it's done, unplug it from the computer and plug it into the non-functioning one
12. Start it up and enter the Bios (if you don't know how to do that, it should display a key to enter setup/bios)
13. Set the boot order so that the USB or External Hard drive will boot first
14. run through the setup of Windows
15. When that's done unplug the USB or External Hard drive and plug it back into the other computer you used to acquire the ISO file
16. When it's plugged in, Format the USB or External Hard drive and Download KMSpico Windows 8.1 64-bit Activator and put it on the USB or External Hard drive
Link:
https://www.mediafire.com/?ub7d323xon7z92y
17. When it's downloaded extract the folder within the Zip file and stick it on the USB or External Hard drive
18. Unplug it and plug it into the computer you just installed Windows 8.1 on
19. Install and run the activator
when you run it, it will appear to do nothing, but when you open the control panel and click system. it will say Windows is Activated
20. (Optional) Finally if you don't like Windows 8.1 Pro then simply install Windows 10 via this link
Link:
https://www.microsoft.com/en-us/software-download/windows10
I hope this fixes it!