That says that you computer is either not booting from you hard drive or your hard drive has lost the ability to boot to the Operating System.
Without troubleshooting, odds are you have a failed hard drive. Fix by replacing hard drive and reloading Operating System
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.