Did you install anything new on your system, a new VGA or some ram maybe? Possibly upgraded your BIOS? In other words was there anything that might have acted as a trigger for the change. That will help me try and fix the problem.
If all else fails, remove every thing from your motherboard, including the CPU. Take the battery out too. And do a clear CMOS leave the board for 12 hours and then try again with only one stick of ram and the bare minimum of hard ware connected.
To help us help you - please provide as much information about your system and the problem as possible.