Try this...
1. First, when turning on the computer, press and hold the "F2" key. You should be taken to BIOS.
2. Go to the "Security" tab inside BIOS and then go to "Secure Boot".
3. Disable "Secure Boot".
4. Now go to "Advanced" and then highlight "System Configuration" and hit Enter.
5. Go to the Boot Mode and change it to "CSM" mode.
NOTE: Whenever you change the "Boot Mode" from "EUFI" to "CSM" mode, another option will come just above the boot mode, "SATA Controller" mode.
6. Hit "Enter" on "SATA Controller" mode and set it to "Compatibility" mode.
7. Now you want to "Save" and "Exit".
That’s it. Now you should be able to boot from a DVD/CD/USB drive and repair your Windows 8 PC with disk.