You need to boot the thing with an Emergency Boot disk and then check and see if you can access the hard drive. If, after booting with a floppy, you can access the hard drive from DOS prompt, you most likely got wacked by either a virus or you have corrupted sectors on your hard drive from a power surge. If on the other hand, you cannot access the drive from a DOS prompt, you have had a hardware failure and your hard drive is toast. Use this page to create a boot disk. You will need to find another computer running XP to do it:

http://www.computerhope.com/boot.htm