Is your "C drive" a separate physical drive or is it one, of multiple partions on a single physical drive?
In either case, do you have the means (skills and equipment savvy) to remove it from its' current system and slave it to another system to see if you can either access it to save your critical data or perform some hardware diagnostics on it like Check Disk or something more exhaustive like Spin Rite (which is not free but rather a commercial, for purchase hard drive utility)?