Sounds like your disk has become corrupted, or has otherwise failed. Disks are notorious for failing, just when you think that they are "running well". The only answer for data redundancy is RAID 1 (or a derivative), plus backing up to an external hard drive.
I suggest using some sort of Partition repair program in an attempt to rebuild the disks "Index" thus potentially allowing you access to the disks contents again. However this sort of activity is risky, either you will fix the partition or make the situation worse, in my experience the outcome is usually 50 / 50 - sometimes things end with a smile, and other times a tear!
Best of luck...