go to the city screen and check if the citizens are working the fields in city radius. Most probably, your citizens are turned into specialists/entertainers - move your mouse over the faces displayed - it should say the status of every citizen. Left-click on the city (in other words - the centre of the city controlled plates) - this places your workers automatically, usually with the priority of food.
if this doesn't help - you are probably in anarchy (in the midst of revolt). wait 6 turns (1 if civ is religious) and implement new govt form - things should return to normal.
other possiblity is that some cities with one spare food, shrink back after they have grown, cuz no more food was around. (i.e. a city of 7 is growing slowly, because it has just one food to spare. eons pass, and it reaches 8, but only unoccupied plots left are mountains. one population point needs no less than two pieces food, but your city had only one. It shrinks back to 7, thus returning to our starting point. In this case, try to irrigate plots with mines (consult c-pedia if that would result in more food).
NOTE: you can irrigate ANY plain plot if you've entered industrial era (or maybe modern era)