Is this a new drive? If so, go to disk management and initialize the drive.
If you don't know how to do that, you can look at my guide: http://www.tomshardware.com/forum/265764-32-guide-installed-disk-system
If it's a drive that you have used before, please provide a brief history of the symptoms and changes when it stopped working. My favorite is "It worked before I took it to France and it was X-rayed in airport security."