originally zor-el & allura were thought to have died along with everyone else in argo city. years later kara discovered that her parents were still alive. zor-el had discovered a "phantom zone" like dimension, that he called the survival zone. when kara learned that she (with help from fred danvers, i believe) she freed her parents from the survival zone. zor-el & allura decided to move to kandor
You can find that story here if you can get a copy,ACTION COMICS 310 (May 1964)