Nomad, I had similar problems like Fletch, but I didnt change cards or even take the card out of the camera and my ML stopped working.
Did 2 sets of pictures the day before 2 x 300 pics and today I did 300 with no problems but after that when I moved to new location and set up everything I just couldnt get ML working. Pressing AV didnt do anything, turning on / off didnt help, battery was on half, I could see free GB in the bottom of the screen but ML woudnt come up. I have build from 13. The only way it would come up on the screen was if I pressed the button under AV Camera one with red dot.. then I would see all that things from ML, and pressing AV then worked.
Just recharged my battery and now ML comes on when I press AV every third time or something like that, but at least its working now.
Should I delete the files inside ML/SETTINGS like you suggested to Fletch or ?