Operating system not found

I have a Toshiba Satellite which was pre-installed with Windows 98 as its default. I upgraded it to WIn XP Home Edition.
I turned it on the other day, and it made this tapping sound, then brought up the message: “Operating system not found”. I entered the BIOS setup and it doesnt detect the hard disk. What can I do? Do I just have to re-install the OS? Or try using a boot disk? I am prepared to start using Windows 98 again. Please help.