I used a service to migrate my members (9K +) called gconverter.
After the migration the Membership dashboard (and the WPMU) dashboard vanished from the back in admin area.
The plug in is still functioning on the front end.
I asked the service for an explanation and they told me that all they did was access the database and enter the users there.
We did this twice and the same thing happened and there was no other work being done on the site so I am pretty sure it was this action that caused the sudden disappearance.