Other, less dramatic possibilities include:
1. The machine has a floppy drive, and there's a non-bootable floppy in it.
2. The CMOS battery died, and the machine has "forgotten" which is the primary boot device.
3. Some other cause (e.g. a failed overclock) has caused the BIOS to reset to defaults, losing the desired hard drive priority or boot order.
4. An internal data or power cable for a drive has come loose.
If it isn't one of those though, unfortunately your hard drive probably opted for early retirement.