Hi,
I take it that the C drive shows up in My Computer, but not in Disk Management? In that case it's probably not a HDD problem. Instead, those symptoms point to the TDSS rootkit. It can often go unnoticed for weeks or even months since it is somewhat hard to detect. I had the EXACT same symptoms when I had the TDSS rootkit: C drive no longer in disk management, other drives no problem. Took me over a month to find and remove it.
You can follow the procedure in the link below to detect/remove the TDSS rootkit:
http://support.kaspersky.com/viruses/solutions?qid=208280684
Make sure that both "Services and drivers" and "Boot sectors" are checked (TDSS often resides in the boot sector).
If the rootkit is removed properly and the machine rebooted, the C drive should reappear in disk management.