A disk boot failure can be 1 of many things:
1. Boot options are not set properly in CMOS
2. Non bootable CD or CD-R in computer
3. All boot devices set are not bootable
I'd say the most likely (if it just started happening out of the blue) would be number 3. Open up your case and see if there's a wide thin grey (IDE) cable and a small white connector with 2 black, 1 yellow and 1 red wire connecting to your harddrive. (If you have a sata drive, instead, you will have a very thin RED cable)
An IDE cable
The one on the right (Molex connector):
Sata Cable
In any case, try unplugging and plugging back in all the cables leading to your harddrive.
If this doesnt help, then perhaps your CMOS was cleared somehow and you haven't set a bootable drive. You can usually enter your BIOS by pushing the DEL button (PUSH XXX TO ENTER SETUP) or some button right as you start up your computer. Here's a list of popular entrance commands (ignore the part about reformatting and scroll down about half-way):
http://www.cyberwalker.net/faqs/reinstall-reformat-winxp/enter-BIOS.html
Once there, you will have to navigate your way into the boot menu: Look for a setting that says, Boot, Boot Order, Boot Sequence, First Boot Device, Boot Management, or something similar and select it. There should be an
Auto-Detect command which should find a drive. Exit & SAVE your bios and try to reboot.
If this doesnt help let us know and we can go from there. Hopefully your data isn't corrupted on your harddrive (which may be the case, and you will need to start from scratch), but we'll hope for the best!
Good luck!