Somone could have removed your access. When you are in the All Compnay group, check out the members area in the upper right. The people with a star are your Admins, you could reach out to one of them and see if they removed your access.
Also, less likely but strange things happen, clear your cookies & cache and relaunch Yammer. Every so often there are temporary overlaps between networks (it seems like you have one External Network).
Thanks Erin. I see that today the Network Admin option is back. I am not sure if this was a temporary issue and there was no alert/notification sent out for this.
