What have you considered to be a possible cause of the problem. Did you do anything that might have removed the partition or the drive letter? Have you added additional drives, or changed the drive connections? Any external drives/flash drives?
Any chance you might have a virus? Do you hear any strange noises when your hard drive runs?
Any unusual behavior or electrical interruptions?
Did you look at the drive with Disk Management to see if the partition was still there but without a drive letter?
Are you running any type of virtual drive or any such software? Any encrypting software or something that might try to hide a partition?
If you think about what has been happening with your system over the last few days, you may remember something that could lead to an answer.