First verify the hard drive failed. Get into the bios and see if the disk is detected. If the BIOS reports the disk is there then we may be able to fix. For example, you might just need to repair the master boot record -- a simple thing to do. Post if the hard drive shows up in the BIOS. Post if you do not know how to check this.
Warranty checkpoint. If you are still in warranty then stop doing anything once you know the drive has failed and call HP. Decide with them if you want them to send you a replacement drive or if you want to bring your PC somewhere to be fixed.
Out of warranty: If the current disk has failed then you'll need to buy and install a new hard drive. This is not too hard and there are good directions on the HP web site. A good 1TB disk drive is less than $100. You can get smaller or larger. You could try a SSD if you want to and you are OK with a small drive. A good 240GB SSD is around $150-200.
Once disk is installed use your recovery DVDs to load the initial factory install on your new hard drive. This is pretty easy. Just put the DVD in the DVD drive. Power off the machine. Power it back on. The PC will boot from the DVD and ask you if you want to recover. It takes an hour or so.
Once you recover your system do go through all the windows update processing to get fix current, especially the security fixes. The only way I know to do this is run windows update, get the fixes, load them, re-boot. repeat. again and again as waves of incompatible fixes are installed.