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: