Hi,
Thank you for posting your query in Microsoft Community forums.
From the problem description, I see that you are unable to boot the computer and receive error message "Missing Operating System".
I can imagine the inconvenience you have experienced. We are here to help and guide you in the right direction.
Provide us few more information to understand the issue better.
1. Are you using Windows 8 Operating System Disc or Recovery Disc to perform Startup repair?
2. What exactly happens when you try to perform startup repair?
3. Do you get any error messages?
Method 1:
I would suggest you to remove all external devices other than mouse and keyboard and check if it helps.
Method 2:
Make sure computer is booting from CD/DVD that you are inserting to perform startup repair. You make check the boot preference in BIOS for the same.
Note: Modifying BIOS/ complementary metal oxide semiconductor (CMOS) settings incorrectly can cause serious problems that may prevent your computer from booting properly. Microsoft cannot guarantee that any problems resulting from the
configuring of BIOS/CMOS settings can be solved. Modifications of the settings are at your own risk.
Let us know the status of the issue after performing the troubleshooting steps.
If you need any further assistance or information about Windows, I will be glad to help you. We, at Microsoft strive towards excellence.