Constance is alive and well. However she is not too happy with the Knots Landing experience and has refused, so far, to be on any of these reunions. The character of Laura was killed off as a cost cutting measure. Knots had been on the air about 7 or 8 years when Laura was killed off and all the actors were making substantial money. The costs to make the show had skyrocketed, but since Knots was never the blockbuster that Dallas was, the ad revenue wasn't quite as high. In fact, the actors let them film their own cars and even their own homes in order to cut location costs to keep the show on the air, they loved it that much.
In order to cut costs, they killed off some characters and brought in new, cheaper actors. Constance wasn't very happy about this decision. She didn't want to leave the show, but had no choice. In fact, if you remember in the reunion on Friday when Greg was crying about 'Red' (what he called Laura) and they showed the TV in the background, Laura's image was fuzzed out. Honestly, I am not even sure that was Constance's voice that we heard. I don't know if that was the producers decision, or if Constance didn't want her image in any way on the show. It is pretty sad, when a beloved character like Laura isn't represented, but if you were angry at your employers and the whole experience, I guess that wins out. Kind of sad really. It would have been great to see her.