Long title, but accurate. This was on a Dell Optiplex 760- dmesg showed only one core being found, cat /proc/cpu only showed one core, etc.

  1. sudo vim /etc/default/grub
  2. on GRUB_CMDLINE_LINUX_DEFAULT, change acpi=off to acpi=noirq
  3. save file
  4. run sudo update-grub
  5. reboot

Now your cores should be found. Leave a comment if this was able to help you.