sarbian's post in No DeltaV Mechjeb was marked as the answer
Usually it mean the default window config got damaged. Open MJ settings window and find the reset button. It should fix things.
Or you can open the "Custom Window Editor", use the top button to select the window you want to edit (or create one) and add the DV (vessel category and "Stage stats (all)")
If it still does not work you will have to post a log.