That means that the computer cannot read the boot record on your hard drive -- could be a dead hard drive, corrupted boot record, or error in the bios.
Easiest is to check the bios first and see if your hard drive shows up there and if it is the first or second (after the optical drive) boot device. If you have a system or repair disk boot from it, and then go from there to determine if you need to repair the OS or replace the drive.