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.
^ agreed. Most of the AMD boards come with a Core Unlocker tool to unlock x2 and x3 CPUs to be a x4... but in your case, if this tool has been turned on, it is possibly deactivating one of your cores.
Usually the first step with booting up is to get into the BIOS and tell it to load Optimum Defaults. If it won't, then clear the BIOS memory by unplugging your PC from the wall and removing the CMOS battery for a few minutes.
If these things didn't help and it is still being reported this way, you could have a defective CPU or defective board.