My pc won't boot up or recognize hard drive.?

So my computer has been running fine for as long as I can remember. I turned it off a couple of days ago when I left town and today when I turned it on after getting back into town it tells me windows failed to start. It told me to insert my installation disk and then restart and ultimately click on repair your computer to fix the problem. I did but when it asked my to select my hard drive/operating system to fix I didn't have any options. It didn't see my hard drive apparently. However, when I run diagnostics then it shows that my hard drives checks out fine. What gives?