It could also be a change to the BIOS. The smallest touch-up in the BIOS will mess everything up. If the above suggestions fail, reboot your Motherboard to Default. Clear CMOS by inserting the jumpers where indicated in the M/B Manual. If you don't have the manual, find out your motherboard manufacturer & model number, get to another computer with internet access and go to the manufacturers website and get the Manual there.
If all of the above fail, and I don't recommned this but it depends on your anxiety levels, look for the text "CMOS" somewhere in your Mobo and set the jumpers where probably indicated. DON'T TURN YOUR COMPUTER ON! Just wait 5 seconds, remove the jumpers and reboot your pc.
Hope that helps!