Did these people actually leave the guild or just stop playing? If they have all actually left, someone has been designated founder. If there was a leader left, that person would automatically become founder. If no leader, the highest ranked person who is active has become founder. Do you know how to access/read the members list that shows guild rights? If they are still in the guild but inactive, the same thing will happen, but I think it takes 30 days.
You can open a support ticket to have an ingame moderator look at this for you.