I remember at one time Ning used to display all of the Ning communities you belonged to when you clicked your name on the top bar, that would be a nice feature to bring back and would help solve problems like your own. Sorry i don't have a simple solution to your problem but perhaps if you emailed Ning using your login email address they could help you.
But... if you still can not locate the network and typing in the url address of the network to your web browser doesn't help, because you see only that the network is "in the maintenance" - it means it was disabled by ning.