I have a Windows 7 dvd but when I try all the methods of repair none of them work. My main OS that is currently 'missing' is Windows 8 with Ubuntu dual boot.
Edit1 after the comment: When I typed that command in the CMD it said it was completed successfully but when I try to boot from my HD it still says "Missing Operating System"
What happens if you replace
NB: this also means your problem is not with the boot sector.