I suppose the first question would be do you know of anything that might have caused the problem?
Have you run an anti-virus checker?
What does Disk Management show about your drives?
It might be advantageous to open an Administrative command prompt and type SFC /scannow to check your system files. Give it some time and let us know if show nonrecoverable file errors.
If something is going on with your hard drives, running Chkdsk might help clear up the problem. There are several switches you can use with it, but I would suggest you first do a check and not a repair to see what it might show. Your next step might depend on what type of errors were found, if any. So, if the partition with the problem was d: you would use...