This is often the sign that the hard drive has failed. The hard drive holds your data (music, email, programs, photos, etc.) so this is a bad thing.
Go into the BIOS on the PC. Go into the menu that shows the hard drive and the CDROM drive. If the CDROM drive is there and the hard drive is not that is a good sign the hard drive failed. If the hard drive is there let us know, there is something else wrong.
Installing a new hard drive is an easy task. Once a new hard drive is installed you need to use your recovery disks to return your system to factory new. If you did not make recovery disks you can buy them from Compaq/HP for $10-$20. The support web site tells you how to order.
Data recovery from a failed hard drive is sometimes possible, but always expensive. Google 'disk failure data recovery' if your disk turns out to have failed and you need to recover something.