when they added in the "super leader" position to the ranking system within an sg, this gave that toon supreme power in an sg, this position when added was automatically given to the first person that logged in on the first day after the patch was added with the rank.
its very very likely that someone else "inherited" the sg by attaining the superleader rank when it was added and possibly removed all the poeple who were inactive or not online after a certain amount of days
finding out who runs the group or who is still active in the group could be like searching for needle in a haystack depending on the how active the group was.
It sounds like you simply got kicked out. You could try posting in the section for the server it's on and ask if anyone can put you in touch with the current leadership.
Paragon City Search And Rescue
The Mentor Project
I've been gone for a long time but came back for the weekend. However when I logged on, all my characters were no longer in the SG I created. I checked the registrar and it's still there, even the SG motto hasn't been changed. Could it have been disabled or removed or something while I was away?