First things, first is there a floppy disk in the drive?
2. Are the hard drive cable(s) firmly?
3. Was there an electrical problem that caused the spontaneous shutdown
}such as a brown out or the power cord suddenly jolted
Sorry I made HUGE grammar error, I meant to say Are the hard drive cables firmly connected. Like cscgal said any media in a drive(floppy, cd, zip) or other could cause the computer to attempt to boot from the incorrect device. To reiterate on what cscgal said to go into the BIOS you usually have to hold the delete or F2 key (BIOS manufacturer dependent) while it is starting then going to the Boot Sequence option will show you which is first. Confirming these three things will show that it is not a BIOS conflict and more likely a hard drive failure (lets hope not).