It seems that any of the old Ning networks that were set up before they moved to the paid for model are being removed.
A number of you have reported getting E-mails from Ning saying
We are going to permanently remove networks from Ning that have never purchased a Mini, Plus or Pro plan. Although networks without a paid subscription are eligible for removal at any time, we’re contacting you today as a courtesy. If you’d like to get your networks back up and running, you have until February 10, 2012, after which time we’ll permanently delete the network(s) listed below and all of their content from our servers.
So if you do want to keep your Ning network, then you have to subscribe to one of their paid for plans.