There are different ways to disable a cpu/core. On of them being passing the maxcpus=N kernel option at boot time. Anyway, check under /sys/devices/system/cpu. In a dual-core system you should have two directories: cpu0 and cpu1. The cpu1 dir should contain a file called "online". If the content of the file is 0, the core is actually disabled, if the content is 1, the core is enabled (online). If you want to re-enable the core, just do
echo 1 > /sys/devices/system/cpu/cpu1/online
then you have to investigate how, who and why the core was disabled.