Well, you cant lose cpu cores lmao, you just somehow disabled them. Reset your bios, also, why you went to mess with the cpu cores? they are always all active by default.
Some people mess with them for x y z reason but... usually you never EVER FOREVER IN ALL EVER need to mess with them.]
furthermore if you really want to mess with your cores... you need to guide us through or learn how to, bios interfaces are different from mobo to mobo, manufacturer to manufacturer, version to version... we cant simply blindly tell you how to .