The drive must've been mountable at some point, or you wouldn't've been able to install Windows onto it. Use your Windows disc to boot to the recovery console, then run the command:
chkdsk c: /f /r /x
Let that run through (it'll probably take roughly an hour or three depending on the drive). If it reports any bad sectors, consider the drive unreliable for the purposes of storing anything important and expect further problems.
If the command doesn't work at all (still can't find the HDD), check the drive is connected properly - a cable may have worked loose. Failing that, try it in a different computer, and if it's still not detected there, then you've probably found the reason the X-Box wasn't booting. (IMG:
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