I double-clicked on the city level up screen picture, the dialog disappeared (but played a building sound), and now my city is stuck as a village with no way to select a level-up bonus and no way to progress in levels. Population tooltip reads "city at max level" and the progress bar does not update, although growth still gets added to the population (note that it is now at 52 after a turn).
I guess it is time for a restart 
