Check in MS config and make sure the core count on boot didnt get set to something, then go into BIOS and make sure that you didnt activate ACC and set it to 3 cores or something. If you havent played around in BIOS and this randomly happened, first try rebooting to see if it comes up right, then try clearing the BIOS by pulling its battery for a bit then putting it back in, that will reset to factory defaults.