It is likely that your hard drive has either died or suffered some significant other issue. If your model has an optical drive try booting from a CD/DVD and checking the drive with SeaTools. You can download an image of SeaTools from
HERE and burn it to a CD using another computer. Then boot your computer from the CD to test the hard drive.
If the drive is bad you will need to replace it and then reinstall the operating system. Hopefully, you have installation disks to do that if needed.