Missing OPERATING system

Hi all,

I got a blue screen error message saying "memory dump". Now when I start my computer it says "missing operating system".
1. I tried to reinstall Windows 7/64 bit with my installation disk. Didn't work.
2. Tried to use a system restore disk that I made when the computer was new. Didn't work.
3.Tried to boot up from an old HD with windows on it. No luck
4. Tried to boot up in the safe mode. Got the same error message.


What more can I do to recover my missing operating system?
How does a computer "lose" an operating system?

Thanks for your help,
Dan