It was still grinding away.
You threw away all that effort.
Unless you are running on battery, let it run (minimized in background). It can take 24 Hours or more for some complex conversion (looks alone don't determine complexity).
Calibre conversions CAN be CPU intense. The fans may run at speed. That is why they are put there, just allow good airflow. Modern computers will protect themselves if the get too hot, but helping the cooling out (and a good de-linting) never hurts
