What motherboard do you have? Could be your motherboard doesn't have the proper BIOS update to correctly recognize the CPU. Tell us more about your setup though as we can't really help you that much without more info.
So did it stop recognizing the second core after you flashed the BIOS? Was it detecting it properly before that? If thats the case, I would roll back and reflash to the previous BIOS version where it was working. Could be the BIOS has a bug in recognizing your particular CPU.
Well today i revise my bios to older one which was originally restored in board but it still show only one core. Last time i have updated my bios through intel itk software. So i think i have to change some options in edit bios file setting of the software.
That board should support both cores just fine. I would try and find the BIOS option to verify that both cores are enabled. If everything it set properly and it STILL isn't detecting both cores, I would guess either your CPU or Mobo is having a problem, though I don't know which, and may have to be replaced. Do you have another CPU or Mobo that you could test to see if the same conditions exist?