Most likely the hard drive has died or the cabling to the drive is disconnected or faulty.. If the hard drive is easily removable like most laptops, try removing it entirely from the machine, then slide it quickly back in the computer to make sure the connections are reseated. If that doesn't work and if you're unfamiliar with how to open and repair laptops, you'll want to take it to a repair shop.

Hope this helps.

Grif