Depending on how old your computer is, your hard drive may have gone bad. This is most likely if it is a relatively new computer or a rather old computer. If your computer is still under warranty, try to contact Dell and see if they will replace the hard drive. Unfortunately, if it is bad then you will not be able to recover your data from the drive unless you want to shell out a few thousand dollars for a data recovery service.
If your computer is not under warranty than you will have to purchase a new hard drive, install it, and reinstall Windows onto it using the recovery disc that came with your computer.
Best of luck!