Sounds like the hard drive has failed and will need to be replaced. If you are still in warranty HP will replace it. If not a retail 2.5" laptop SATA hard drive can be used. Just be sure to get one of at least equal capacity as the original in order to use HP Recovery Media to install Windows. It can be larger, but not smaller capacity.For instance if you have a 500GB hdd you could use a 750GB hdd but not a 320GB hdd.
If you need more help selecting/replacing the hdd post back with full Model & Product Number as listed on the Service Tag attached to bottom of laptop.