Well you have 2 probable reasons for this, either A. Your system doesn't detect it, or it was inadvertently unplugged. B. When you installed windows 7 you gave it the entire hard disk and it now only has a single available disk, disk C: by default.
Right click on Computer and click Manage, then select disk management and see if any other disks show up other than your current one and your cd drives.
Many partition softwares can solve this problem since you can unhide partition with them. I encountered the similar problem before. And I downloaded a free magic partition tool to solve it. You can go to http://www.magic-partition.com/?8 to download first and install it. Then you can experience the cool magic partition right away. Good luck.