Did it before? What occurred leading up to the disappearance? Is it the primary HDD (boot for OS) or a slave or an external? What is it's model and how does it connect? Does it 'spin up'? Any other details...like loud sounds on start, grinding noises, beeps, clicks, squeals? What flavor Windows? How long installed? Any other system failures? Any important facts (like it dropped to the floor, etc)? Please check Event Viewer for any interesting 'errors' (skip restarts after lost power, unexpected power downs and OOBE).